A relatively easy step to take would be to feed yourself when you feed the children.
there are adhd workbooks for house cleaning. It’s like a timetable thing with daily tasks weekly tasks monthly tasks etc with tick boxes. You may not be adhd but something like that could help. You could add shower and brush teeth etc
sort yourself a routine. Washing. Meal planning shopping (get online groceries delivered). I recommend long life milk and having spare bread in the freezer. Even if you do a two week meal plan and always just do that if it’s a balanced diet you just stick to that and think about it less. Learn some easy things you can cook extra of and freeze- eg bolognese so you just have to microwave that and cook the pasta on days you run out of bandwidth.
bills are a faf to set up if you split but if you set up direct debits you don’t have to think about them if you stick to your bujget. I don’t know if CAB can help with that. There are money management charities that help people in debt who may be able to walk you through it. Or adult social care.
alarms for medication.
right now you stop trying to save your relationship. You don’t have the energy for that. You need to focus on how you can manage alone. In doing that you may find it shifts some burden off your partner and that’s what he needs.
you said your mum can’t take you in because she has a lodger. If she didn’t have the lodger would she be happy to? Because if you get pip and you will get child maintenance if you split and the kids stay with you, your mum could claim carer allowance. Possibly this will make up the shortfall from the rent? Talk to her- you don’t know until you ask.
your children are very small. You will be knackered even without autism in the mix. Things do get easier as they grow up generally.
I’m autistic and my son is high needs autistic, I felt like I was breaking and couldn’t cope when he was 3. I can manage all the tasks I’ve listed here, work and function fine. But when you are struggling everything becomes so much harder! I started the meal planning same meals on rotation, washing was Monday washed and hung, Wednesdays I put the washing away, Sunday I cleaned bathroom and kitchen and changed beds. Just routine so things didn’t overwhelm me, and I e kept that up because why take back extra mental effort when it works well?!