Oh @Milkminder, it's all coming out now isn't it? I hope those who were quick to jump on early and deride you as a poor mother who should never have had kids will join some of those who have back-pedalled hard.
When you got together with your DH, presumably there was an expectation that you would carry the household, because that's the background he came from? But that doesn't mean you have to continue the work of his mother in allowing him to live the lifestyle of a man in the 1950s!
You haven't mentioned your family at all. Are your parents still in the picture? Did you grow up in a household like this?
It sounds like you love your children dearly, and you really do give an enormous amount to ensure they are well balanced and cared for.
But no-one other than your husband and his family would expect you to do all you do. Not your GP. Not citizens advice. Not a family judge in the divorce court. Not anyone in support services at women's aid.
Maybe have an online chat with them? chat.womensaid.org.uk/
Or book an appointment with your GP to explain you're at breaking point. You can discuss that you seem to be doing remarkably well and aren't coming asking for anti-anxiety or depression medication, but that time talking to someone to boost your self-confidence and build skills to improve your situation would be the best medicine of all. Counselling services are incredibly stretched at the moment, but I would remain hopeful that a conversation with a good GP might mean you are given support to access some local services to build your confidence in the fact that - since you're doing all this on your own anyway - you might be better off letting the third child go running back to Mummy so you don't have to carry his dead weight through your life as well.
I'm really hopeful that you can reconnect with some friends in person, go for a walk with them and talk this through, even if it's with both kids wandering around the park with you on a Saturday at the moment. This pandemic has been incredibly isolating, and emotional abusers love to isolate partners from their support networks. It makes it far easier to pass off the fact that you are their 24-7 skivvy as 'normal'.
I think you've had a real awakening through this thread that your situation is not the norm. And it's not your children's fault you are resenting it. Try to find some local in-person support to talk this through with.
If you stop cooking and cleaning for him and he runs home to mummy, how much worse would your life be? Getting a line in the sand with someone like a GP to document the fact he doesn't lift a finger would help if it did come to divorce. Would you be able to survive financially for now if he did move back to his Mum? It will help if you can get it on record in a couple of places that he does nothing to help. Then stop doing some of it and see what happens.
You deserve help with half the work, or half the money. Please don't let his mother's approach to life grind you down for another generation.
Have a read and a think 

www.womensaid.org.uk/information-support/what-is-domestic-abuse/women-leave/