If it makes you feel any better I was there, too. Couldn't keep my mouth shut and it doesn't work. Bite your tongue, bite it, bite it. Even if you think you're right about something, it's not worth it to "just be right" it's the booby prize, because all you get is to be right. You may feel better but your H won't.
As hard as it, back off and get on with your life, it's hard at first, but it gets easier, it'll make you feel better and the better you feel the easier it is to let him comments, words, reactions role off your back.
I went back and forth so many times with a lawyer, also, especially after H and I had had a hard time, but I'm no longer going that way nor is my H. Don't talk R!
It sounds like your H doesn't know what he wants. Did you say there is OW involved? Maybe she's pushing him. And your MIL is right the grass isn't greener on the other side, if he doesn't solve his problems now they'll just carry over into his next R. And, if he doesn't start shaping up do you want him back anyways?
