I agree with Splitimage, tell them you are having medical issues and you need to work 1/2 days, making it 20hours a week, or go on disability for a couple of months than try going back 1/2 days. It's all how you feel and what the company is willing to handle. I've been where you are now, calling in sick all the time and I felt my job was on the line, but I'm on disability do to my headache and now for depression.

Good luck on what you decide, it will get better.