And, I don't want to sound childish, BUT I DON'T WANT THERE TO BE ANY TEXT MESSAGES TO READ.
I don't blame you! This nonsense that you should just accept his OW is ...well...TOTAL BS!
I know you have stayed because of your twins still being in high school - are they graduating this year or next?
Let's face it - IF you had unlimited financial resources, you would have booted him out a long time ago, and he would probably have woken up out of his fantasy pretty quickly and come crawling back. But because of your financial situation and desire for stability for the kids, he's got you by the short hairs and is having his cake and eating it too.
You have an awful lot on your plate - your kids have a lot of special needs, and your H is just sucking up your energy and pulling it away from the kids, where it really needs to be.
There's no rule that says you have to put up with his bad behavior, or that you have to keep standing for a man who so blatantly disregards your needs. Just be smart and ask yourself about every decision "Will this get me closer to my goals?"
And take advantage of his current (probably short-lived) helpful mood to get EVERYTHING you need done around the house. If you DO end up divorcing and selling the house, it will be that much less you have to do to get it ready to sell. Even if you keep the house, it'll be that much nicer for you.
Sock away any money you can in any secret places you can. Try to get to the point where you COULD, IF and only IF you wanted, tell him that refuse to be in a triangle any longer and he has to leave. I'm not saying to do that - that's a decision only you can make. But wouldn't you feel a lot better if your ducks were in a row so that this WAS an option, if you wanted it?