OP. Stop, just stop. Re-read your own posts, in particular the one where he says he wants to go out with her & ANOTHER couple.
He and her are the couple. You and he are over, I suspect he's been getting his ducks in a row & also enjoying the lower he has over you.
Yes, he's been emotionally manipulating you, but you know that now.
Time to stop the naval gazing & get to a shit hot divorce lawyer. Do it NOW.
Your marriage is over.
Do not move out.
Get std checks.
Play along. Let him go out with her & their mutual pals. Emotionally disengage.
use the evening to
Copy all bank statements, pensions, mortgage paperwork, Isa etc EVERYTHING you can think of, rummage through where he may have hidden anything (new accounts, etc)
Pack away anything special.
Head over heart until you have a lawyer in situ. Then serve him the papers.
You're only 35
Let her have him.
You really really are better off & young enough to. Start over.
Don't get older & more manipulated & insecure whilst he shags his tart on the side.
Take back control
🙏🏻 keep posting if you find yourself wobbling.