OP it sounds like you are feeling absolutely overwhelmed and it’s entirely understandable. You have been an absolute rock for your kids. I think it would help if you carved out an hour of time for yourself to sit down with a piece of paper and try to separate out all these issues, write them all all down, then look at what you can control.
The children - it sounds like you have done an incredible job with them, but also you are exhausted and need more support here. That could come in the form of financial support through DLA as others have suggested. I don’t know enough but it’s worth investigating surely. Aside from that are you sure you are utilising all the support the school offers? Worth thinking about.
With the house I sympathise- we live in a tiny 2-bed terrace with no hall, sounds very similar. Albeit my DC aren’t full size yet. Again I would get your piece of paper and write down all the issues. What are the worst aspects, hotspots, things that grind you down every day. You mentioned DIY. (Your husband sounds pretty useless by the way, and I’m sorry for you). But what can you do with these DIY jobs? Can you do them yourself or get someone skilled in the blitz them for a morning? It might cost a bit but could be a great investment for your mental health if that dripping tap is finally fixed and the damp patch has been painted over?
In terms of how the house works for you, can you do a massive declutter? Again investing some time for your future mental health and to regain a sense of calm - it would be worth it. If there a particularly bad area can you re-jig things to make it work better? Eg we put new hooks up behind the front door in our living room and put the current jackets in use there, and the rest away in the cupboard under the stairs, which in turn got a revamp with some fitted shoe storage and hooks. We put very slim shelving against the wall by our front door, one side of our living room with books and the lower shelves were for shoes and bags (hidden in baskets) so it keeps the entrance way clear for people to walk through.
I also spent ages looking up tiny home storage ideas on Pinterest for my DC room and make the most of the space (after having decluttered everything). Sounds trivial perhaps but it really made a difference.
I also did a cheap refresh of paint/ cushions / plants etc to make the place feel nicer.
the pp idea of clearing out your back yard and adding a shed is great- a teen hangout perhaps.
I also think you should remind yourself of the benefits of your place- the excellent location, the lovely outdoors. Do you get outdoors much? Do try and make the most of your beautiful area if you can as this will really help your stress levels. Maybe a daily walk or run before anyone else is up.
With your husband I’m so sorry he has been so crap. This is a bigger deal. TBH it doesn’t sound like you have capacity for dealing with your marriage just now it I think if you can take some of the above healthy steps you might start feeling better / calmer in yourself and better able to tackle things.