St Andrews Lynx

There are some PI out there who will only like an idea if they themselves thought of it. Even though they might solicit ideas from you and insist you should think up your own proposals. I'd say that your PI has less of a "bad" memory, more of a "selective" one.
 
You can't really change an advisor like that.
 
Your best approach (if you plan to stay in the group) is to (i) shake off the random criticism, which isn't a reflection of you or your capacity as a scientist (ii) minimise your contact with this PI and try to find other ways to get help for your research problems (perhaps from other group members) (iii) accept that you will have to "plant" ideas in your PI's head, rather than suggest something and get immediate approval/acknowledgement. I'm not pretending that is easy.

A handwritten card with a sincere thank you message inside would be enough. 
 
Maybe some memorabilia from the place (university-branded, or something ubiquitous to the state) you got accepted to? 

The problem is with *them* and *their* insecurities. It has nothing to do with you. Grad school is great at bringing up all a person's self-doubt, fears and weaknesses, in part because it is much less structured than undergrad. To combat their own insecurities, people bitch about others for working too hard. Or not working hard enough. Or wearing pink cardigans all the time. Or whatever. 
 
You say that you have some friends in the cohort? That's great. Focus your energy on them. It is possible that once your cohort move away from the coursework and get more settled in to their own research that they'll calm down a bit and stop with the nastiness. 

There's also the option of taking a medical leave of absence, which would allow you to return to the program and finish it off at a later date. It also gives you some leeway: if you decide in a few months that you really want to go back and finish the Masters...then you have the freedom to do so. But likewise, if you decide that you don't need this degree, then you have the same amount of freedom to withdraw completely. 
 
At the end of the day: (i) You are not your illness (ii) There is so much more to "success" than the lines on your CV. I hope things work out for you.

The problem is with *them* and *their* insecurities. It has nothing to do with you. Grad school is great at bringing up all a person's self-doubt, fears and weaknesses, in part because it is much less structured than undergrad. To combat their own insecurities, people bitch about others for working too hard. Or not working hard enough. Or wearing pink cardigans all the time. Or whatever. 
 
You say that you have some friends in the cohort? That's great. Focus your energy on them. It is possible that once your cohort move away from the coursework and get more settled in to their own research that they'll calm down a bit and stop with the nastiness. 

There's also the option of taking a medical leave of absence, which would allow you to return to the program and finish it off at a later date. It also gives you some leeway: if you decide in a few months that you really want to go back and finish the Masters...then you have the freedom to do so. But likewise, if you decide that you don't need this degree, then you have the same amount of freedom to withdraw completely. 
 
At the end of the day: (i) You are not your illness (ii) There is so much more to "success" than the lines on your CV. I hope things work out for you.

A handwritten card with a sincere thank you message inside would be enough. 
 
Maybe some memorabilia from the place (university-branded, or something ubiquitous to the state) you got accepted to? 

There's also the option of taking a medical leave of absence, which would allow you to return to the program and finish it off at a later date. It also gives you some leeway: if you decide in a few months that you really want to go back and finish the Masters...then you have the freedom to do so. But likewise, if you decide that you don't need this degree, then you have the same amount of freedom to withdraw completely. 
 
At the end of the day: (i) You are not your illness (ii) There is so much more to "success" than the lines on your CV. I hope things work out for you.

It sounds like you want to leave very badly and are having a hard time to giving yourself permission to do so. It's ok to leave... You're not a failure. I just finished my masters and someone in my program has MS. She had a rough time but did really well anyway. She has learned how to deal with it and takes Ritalin to help with being so tired.

If you're miserable then it's fine to go home and learn how to handle your illness. You did well in undergrad. If you address learning to deal with your illness in you SOP then i'm sure future programs will understand. Decide what is best for you and your happiness and do whatever that is.

If you see the faculty in person on a very regular basis (e.g. you TA for them, you are in their seminar class) then I would tell them in person. If you can go weeks without bumping into them, I would write them an email. 
 
Be polite and concise. Thank them for allowing you to rotate in their lab, tell them that you enjoyed the experience. All you need to say is that you decided to join the lab of Professor X. You don't need to give reasons or make excuses. 

I'm so glad I'm not 19 anymore.
It is so hard to understand how you over-analyze and overthink and have extremely hazy views about the behaviors of those whom you "love."

This isn't love.
He isn't interested in you.
And this specific action sounds narcissistic and rude (Or could indicate that).
You do not (and no one does) deserve to be treated disrespectfully.

You will empower yourself and feel stronger and more desirable (and will be) if you just cut off all communication with him and "walk away."

Also, consider this a huge huge HUGE bump of the post with the advice about "chronological first" vs. "real-love first."
Your "real" first in this sense will be so much better, and you'll both laugh and cry about your chrono first. Hopefully with a loving, caring, and committed partner at your side.

When I was a bit younger than you, I had a similar "relationship" with a guy, who I was basically in love with, and I found out he had a bet going with his friends about whether he could de-virginize me.
Fortunately I found out before it ever got there.

While you're this young, stay away from age gaps of more than a few years. Those who are jerks are also more clever with age. It's a dangerous game and not worth it, especially when you're an emotionally-attuned person.

I was being tongue-in-cheek.
In case that wasn't clear.

The problem is with *them* and *their* insecurities. It has nothing to do with you. Grad school is great at bringing up all a person's self-doubt, fears and weaknesses, in part because it is much less structured than undergrad. To combat their own insecurities, people bitch about others for working too hard. Or not working hard enough. Or wearing pink cardigans all the time. Or whatever. 
 
You say that you have some friends in the cohort? That's great. Focus your energy on them. It is possible that once your cohort move away from the coursework and get more settled in to their own research that they'll calm down a bit and stop with the nastiness. 

The problem is with *them* and *their* insecurities. It has nothing to do with you. Grad school is great at bringing up all a person's self-doubt, fears and weaknesses, in part because it is much less structured than undergrad. To combat their own insecurities, people bitch about others for working too hard. Or not working hard enough. Or wearing pink cardigans all the time. Or whatever. 
 
You say that you have some friends in the cohort? That's great. Focus your energy on them. It is possible that once your cohort move away from the coursework and get more settled in to their own research that they'll calm down a bit and stop with the nastiness. 

The problem is with *them* and *their* insecurities. It has nothing to do with you. Grad school is great at bringing up all a person's self-doubt, fears and weaknesses, in part because it is much less structured than undergrad. To combat their own insecurities, people bitch about others for working too hard. Or not working hard enough. Or wearing pink cardigans all the time. Or whatever. 
 
You say that you have some friends in the cohort? That's great. Focus your energy on them. It is possible that once your cohort move away from the coursework and get more settled in to their own research that they'll calm down a bit and stop with the nastiness. 

The problem is with *them* and *their* insecurities. It has nothing to do with you. Grad school is great at bringing up all a person's self-doubt, fears and weaknesses, in part because it is much less structured than undergrad. To combat their own insecurities, people bitch about others for working too hard. Or not working hard enough. Or wearing pink cardigans all the time. Or whatever. 
 
You say that you have some friends in the cohort? That's great. Focus your energy on them. It is possible that once your cohort move away from the coursework and get more settled in to their own research that they'll calm down a bit and stop with the nastiness. 

If you see the faculty in person on a very regular basis (e.g. you TA for them, you are in their seminar class) then I would tell them in person. If you can go weeks without bumping into them, I would write them an email. 
 
Be polite and concise. Thank them for allowing you to rotate in their lab, tell them that you enjoyed the experience. All you need to say is that you decided to join the lab of Professor X. You don't need to give reasons or make excuses. 

If you see the faculty in person on a very regular basis (e.g. you TA for them, you are in their seminar class) then I would tell them in person. If you can go weeks without bumping into them, I would write them an email. 
 
Be polite and concise. Thank them for allowing you to rotate in their lab, tell them that you enjoyed the experience. All you need to say is that you decided to join the lab of Professor X. You don't need to give reasons or make excuses. 

Although a contract wouldn't be the norm in my field, having an email conversation with your PI about authorship is always a good idea because it leaves a paper trail. Your advisor can verbally agree to anything, but may change their mind, backtrack or forget they even had that conversation with you! 
 
After a face-to-face conversation about authorship, I would advise summarising the chat in an email and sending it out to all involved parties "to clarify what was discussed". 
 
As soon as your PI starts talking about "publishable data" it is acceptable to start making enquiries about expectations for authorship. "What would I need to do to be listed as an author? What would I need to do to be listed as first author?" Etc. That way you have a good idea in advance and it won't come as a surprise 6 months down the line to find out that you and your PI had differing ideas about your position on the author list. 

At my undergrad university (in a foreign country, so maybe not completely applicable) professors weren't allowed to take on volunteers without paying them. Students had to be officially hired (and officially paid) for university insurance/legal purposes in case of a lab accident. In the US, I know that some science-based fellowships not only cover the student's stipend, but give money to the PI to cover the cost of the lab supplies they'll use, etc. 
 
Working in an academic lab environment - even at grad student level - means compromising on some things. You cannot necessarily just do your own project (it is PI dependent how much input you have in project design), nor can you necessarily do everything alone. This is a temporary summer project, not your thesis. 

This is a followup on question that you asked yesterday. There is no need to start a new thread when there is a discussion going on in the old one, so I have merged the two discussions. 
 
I would suggest not reading insults that aren't there into other people's words and actions. This professor doesn't owe you anything; he is offering to hire you as a research assistant and pay you, which would indicate a favorable opinion of you. He apparently has enough work for a second research assistant. I think your interpretation of this as him "offering to hire" someone to help you is just misguided. If he wants to hire someone else, it's not an "offer" and it's not to help you (because you aren't good enough), it's because there is more work to be done than one person can do (or: that one person can do within the timeline that the professor would like it to be done), and he has the money to hire a second person. This implicates exactly nothing about your own abilities. It's not an insult.

NPR and high-quality newspapers/magazines (New Yorker, etc) are always a good reserve of small-talk fuel, especially in response to what people are telling you about themselves ("oh, you teach kindergarten? I heard on the radio the other day that they are implementing [interesting early education pilot program] in Colorado, do you think that would be a success?"). Asking people about the TV shows they are watching or recent films they've seen is another good way to find common ground. 
 
From talking to people from all walks of life, it actually seems like the teaching part of my grad school life is what others can relate to. They might not understand the intricacies of my chemistry sub-sub-field...but they can understand and empathise with my teaching responsibilities. They also don't need much prior pedagogical knowledge to understand classroom anecdotes.
 
The keys to interacting with folk from different social situations is (i) respectfulness (just because they dropped out of high school doesn't mean there is nothing you can learn from them (ii) a sense of humour (iii) interest in what they are saying. That way even if you don't say much during the gathering, they will still remember you as a respectful, interested person with a sense of humour.

NPR and high-quality newspapers/magazines (New Yorker, etc) are always a good reserve of small-talk fuel, especially in response to what people are telling you about themselves ("oh, you teach kindergarten? I heard on the radio the other day that they are implementing [interesting early education pilot program] in Colorado, do you think that would be a success?"). Asking people about the TV shows they are watching or recent films they've seen is another good way to find common ground. 
 
From talking to people from all walks of life, it actually seems like the teaching part of my grad school life is what others can relate to. They might not understand the intricacies of my chemistry sub-sub-field...but they can understand and empathise with my teaching responsibilities. They also don't need much prior pedagogical knowledge to understand classroom anecdotes.
 
The keys to interacting with folk from different social situations is (i) respectfulness (just because they dropped out of high school doesn't mean there is nothing you can learn from them (ii) a sense of humour (iii) interest in what they are saying. That way even if you don't say much during the gathering, they will still remember you as a respectful, interested person with a sense of humour.

Write a long & detailed letter to your advisor telling him how he made you feel, cataloguing all your frustrations and all his failings...then burn the letter. 
 
Another (additional) option is to send them an email or write a nice card thanking them sincerely for their support and assistance in completing the program. Short, sweet, lying through your teeth if necessary. Give them a small gift (maybe a Starbucks gift card or something under $40 that they would appreciate). Sometimes killing 'em with kindness is the best way to get release, or maybe reaching out with a bit forgiveness will help your ex-advisor thaw and perhaps even volunteer an apology. It certainly won't do you any good to hold on to the bitterness and anger.

It has to be about objective reasons and about you, not the PI. The best way to talk about it is to say that you've learned through doing this current rotation that your interests have changed and that they are now more in line with the work you were doing in your previous rotation. Make sure you say (and repeat) how grateful you are to have been given the opportunity to be in your current lab and how much you've learned. Thank the PI for all he's done for you, mention nothing about his personality. If he is great but you want to do research he doesn't support, it's much easier to make a clean break than if you make it about your personal fit.