doctor-to-be

I don't think that this is bad. I understand how it can feel anxiety provoking. You certainly don't want to risk getting a poorly written letter. Just use your best contacts.

I would do a bit of all the above. Be sincere and genuine, as if you're not, she'll probably see this. I would do exactly what she asked you to do. in this letter, certainly emphasize your weaknesses. Consider yourself lucky that you are allowed to do this. Most applicants are not is my guess.