I hear you. Yes, being burned out as a single parent is a thing.
I'm in the same boat, my DD is 13 now and Autistic/ ADHD as am I. As I get older (I'm 53) it gets more tiring and I suffer with Autistic burnout. I've not had a child free night for over 8 years. This is because my parents are now elderly and my DD won't stay with anyone else. She will only be with me and even sleeps in my bed. I had to give up work in the end, mainly because I'm disabled, but I couldn't do regular hours due to her needs, appointments, admin and dropping everything to rush to school when things go tits up.
So I definitely hear you. As far as advice goes , I've been offered help via our local Autism society and also via our Local Authority. It isnt something that would work for us, but might for you (usually involves your DC signing up for projects with other Autistic kids during summer hols or weekends etc which gives you a break, or a carer to take DC out fot the day) You can contact them regarding having a parental assessment to see what help and support they could offer. I haven't taken up this offer, but it came about as part of my DDs EHCP Needs assessment process.
It's so draining. Sending gentle hugs. The only other thing is trying to make them understand that 'Mum just needs 5 minutes' and going into the garden for a breather. I usually have to say this about 10 times as DD follows me.
Also, do tell school that you're struggling as they are often able to offer support (even if it's just to allow you to offload)
It's so hard, and so restricting. You aren't alone, though 