I did the same thing recently. I had a very stressful position for several years and it kept becoming more and more difficult to handle. At the nudging of my therapist I started looking for something different and was offered a less-stressful job at the same pay in a different dept. I am really scared about starting over at a place where I have no clue what is going on but I hope I made the right decision for me. If I found myself being given more than I bargained for, I would have a chat with my boss. Maybe she just has more faith in you than you do in yourself (which is the story of my life). Then again maybe your boss is trying to do what is easiest for her. Just be open and honest...but don't necessarily expect her to totally understand the effect the illness has on your life and work. Good luck with your new adventure!