So sorry to read the situation you are in. In my life I'm caring for a young adult with severe mental health difficulties. Single mum, working full time from home. I'm really really tired and feeling stuck and resentful.
I'd agree strongly with what previous posters have said about respite care or counselling in any way possible. Are there any peer support groups too for other parents caring for children with similar disabilities, even online? Even online chats with others who get it can make a big difference. Counting small glimmers.
What I have started doing in the last month is beachcombing and cleaning. Whenever I have a spare 40 minutes or hour, walking on our local beach. Its not a glamorous one, just on the edge of our city. I take litter pickers and a bag, gather plastic and fishing nets and twine, see what else I can find and walk by the waves. It revives me. Helps me see progress and a small small bit of difference I am making, even if I can't see it in my own life. And the waves and wind calm me, I like looking at the shells, watching other peoples dogs jumping in the waves and yesterday I found an eggcase, which when I brought it home and soaked it, turned out to be the eggcase of a spotted ray. (Also found a tin of paint, various nappies and a shoe.)
In previous times when things have been hard I've got alot from journalling. I wonder if that might be possible right now? Just writing down all my thoughts as a braindump helped me hear myself, even when I felt no one else was listening, and was a first step to getting back in my body. Julia Cameron's book The Artists Way was a great help here.
All the best OP.