Struggling at work

Hi, new user here so I'm not entirely sure if I'm doing this right...

I have Asperger's syndrome as well as CFS/ME, depression and anxiety. I recently started working again at a clothing shop and while my original manager was fantastically supportive and made a lot of adjustments for me, she's been sent to help another branch and the co-worker who's been made temporary manager isn't quite as understanding.
During my interview I made it clear that I couldn't work for longer than 4 hours at a time and that I usually need a day between shifts to recover, the original manager assured me that I would never be working more than 3 hours at a time and has made sure I only worked 2 to 3 shifts a week unless there were extreme circumstances.

I understand that at the moment we are very short staffed, but the new manager now has me working 4 hour shifts, which I agreed to as long as I wouldn't be working 2 days in a row. However she's slowly been adding two days in a row and is trying to convince me to work 5 hour shifts. By the end of a 4 hour shift I am in extreme pain and usually struggle to walk or stand straight, I lose control over my hands and I can't think properly. I usually get home and fall asleep on the floor because I can't get upstairs. I've tried explaining this to my new manager but it just doesn't seem to sink in, I've told her that if I work for 5 hours and then have a shift the next day I'll probably have to call in sick because I wont be able to work, but it either slips her mind or she's hoping I'm too much of a pushover to say no (which I kind of am). We get on really well in general but she's got a very matter of fact way of talking which intimidates me sometimes and I'm scared to bring the issue up with her in case she turns on me and our working relationship turns sour, especially since I would be working with her alone all the time.

Does anyone have any advice? I know that legally she has to make reasonable adjustments for me, but I really don't want to bring that into the discussion unless I absolutely have to...