I did a similar thing at the end of last year. I spoke to my doctor first and he was very sympathetic. He wrote me a letter to give to my employer. I arranged a meeting with my line manager and we talked over the situation for an hour or so; I handed him the letter from my G.P. as back up. After a few weeks we had a follow up meeting and it was agreed on both sides that I would reduce my hours from 40 per week to around 20 hours per week. I had to work one month full time until they got someone to work the other half of my shift.
If you talk honestly and openly about your situation, and back it up with a medical letter from your doctor, they should do their best to accommodate you. Good luck.