It can feel really devestating to have to cut back or drop out, but what you are dealing with is a real challenge that has a huge effect on your ability to function. I think of it as trying to box with one hand tied behind your back! Please don't beat yourself up or blame yourself for what you can't keep up with, (I used to do that heaps) rather, congratulate yourself for each thing that you accomplish against the odds.
Talk to disability services and your faculty advisor and get the help that you need, then do what you can to get through, one foot in front of the other. Somoetimes just getting through it is good enough.