I just finished my 3rd semester, and my lab few classes and will be performing full time research now and I am extremely excited. My personal program is a bit difficult than the rest of my cohort in that my lab is not on the main campus, but I still have to attend data presentations and seminars as they do, so our experiences are similar. With that said, I am very much looking forward to just going into the lab and being able to work most of the day, without having to constantly plan experiments around coursework, especially since in my program you took two semesters of around 13-15 credits in addition to lab, so there was a serious problem with balancing lab work and research with coursework and studying. I guess I am in the minority where I prefer the freeing feeling of solely research. I am free to plan my week around all of my experiments. It helps that I keep daily, weekly, and monthly goals for myself, since I present in lab meetings once a month. This helps me stay on top of things when my time is free and open. When I am in charge of my own schedule, I find myself working crazy and/or long hours, and I am completely fine with that. Setting my own deadlines as opposed to forced deadlines makes me feel better.
As far as hitting a wall or feeling like quitting, it helps me to strictly stay on a schedule for working. I wrote a review for a book this semester while taking my remaining coursework and working on preliminary data for my thesis proposal, and I was EXHAUSTED. My brain was on material overload. I haven't had any feelings of hitting a wall or quitting with this lab, but I did when I was in a nightmare of a rotation while taking the hardest courseload I have ever had and being required to attend seminars and presentations, wondering if putting myself through this stress was worth it. I just had to remember why I applied to grad school in the first place, all the money, time, and worry put into my applications and interviews, and the joy that I felt when I chose my dream program and dream lab. Now that classes are over, I have put myself on a schedule to work 6 days a week, and completely rest for one day, unless I have a major deadline. This gives me the time that I need to recharge and refocus, get my errands run and keep my home in order. This works well for me, and I don't feel so overwhelmed all the time.