Unfortunately, if a company doesn't want to give "reasonable accommodations", they don't have to. I'm not sure how they get around the ADA on this one, but it happens all the time and the ADA may or may not protect you.

I don't blame you for wanting to hang in there and work just a few hours per week rather than go on SSDI. Work is usually better for us than sitting at home on assistance, unless we really CAN'T handle working (and some truly cannot). I've had to go to part-time myself, as I can't hold it together anymore working a 40-hour week.
I lost my nursing management position earlier this year due to a bipolar meltdown---I went out on FMLA for almost a month, and when I came back they wouldn't accommodate me, so it was bye-bye, Birdie.

Believe me, if your employer's going to be a butt about your wanting to go part-time, you don't want to work for them. Your best bet would be to look for another job NOW while you still have one---it's much harder to find a job once one is unemployed.

Best of luck to you. I'm sorry you're going through this situation.