I know exactly how you feel I had to come to that very difficult choice myself and it took me a year to make the choice so don't feel bad for not knowing what to do. I just handed in my resignation this past monday and it's been a difficult week I really love my job and everyone around me there and it hurt to have to leave. But the new stuff the doc's putting me on is really rough and I know I couldn't work. I've been struggling with the choice for a while and it's not an easy one to make.
I would consider at least cutting your hours, and see how that goes and if you still feel awful then I would talk to your doc about quitting or disability.
Hope you feel better and good luck with everything
Lots of hugs!!!!!! Stefanie