Oh SSD, I feel for you so much! Am in the same boat exactly but a lone parent with no family anywhere near except elderly increasingly frail mother. I have to take annual leave from work if I want to do anything on my own (hairdresser, shopping etc) otherwise I'm either at work or with the kids 24/7. And no-one to look after them, take over when it's tough or have them overnight etc. I truly feel your pain.
Btw, the above isn't in any way an attempt at one upmanship, but just to say I do know what it's like to feel so trapped. It is incredibly draining when you have no space or time to yourself - my two are very young so at least they are usually asleep by 9 at the latest, so I get some kind of evening.
Okay, here's what I do to try and stay sane:-
Make sure I lock myself in the bathroom once a week when they are asleep with a bubble bath, a drink and my smartphone loaded with an episode of Sherlock (or a good book).
If things really get pressured, I get us all out of the flat and into the nearest park - getting outside always saves me from a brain explosion! Just breathing fresh air sometimes is enough to turn things around.
I phone my closest friends and tell them how I feel. I then ask them if we can arrange some dates to get together with the kids, just for some company and someone to talk to. Even if we meet up at the local shops, it's still a change and a chat and a coffee, and a change of scenery. Often enough to diffuse what can feel like an endless weekend. And to be honest, if your friends know how badly you feel, they'll be happy to meet up/come over/have you over. If I don't see my friends for a while, it's usually because of busy lives or it didn't occur to them that I might want to do something - let them know what's going on!
I have an arrangement with a good friend that we ring each other every day or every other day just to offload the parent stress - it always leaves us both feeling better and less isolated!
I stock up on some good films, I joined Netflix (very cheap) as I am a consummate saddo and have my computer linked to my iPhone and I watch movies and TV in bed.
Approximately every two weeks I deliberately watch sad movies and let myself have a really good howl and cry, just to get it all out of my system
I try and do everything I can to take care of myself - half the battle is keeping yourself fit and healthy and getting lots of sleep. (this usually conflicts with my films in bed theory!). I find that if I have enough sleep i can cope with everything but if I m tired, it all falls apart in a quite spectacular fashion.
Remember, You are very brave and very strong - sit down with your DH and work out some ways to give yourselves the space you so desperately need. Be kind to yourself - and remember that there are quite a few of us who truly understand how you feel - you're not alone in going through this and feeling like this - it is one of the hardest things in the world, but I believe it will get better for all of us
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