I dont know if it would help from the other perspective (sort of ). My Ex MIL was awful to me and Exdh and our DS1. Truly vile in a variety of ways.
To boil everything down to ease of writing I got to the point of ending my marriage over her mainly (there were other issues) and it came down to this. I needed my Exdh to put me and DC first and to the other side of it it felt as though he wasn't prepared to face his own issues and do the work he needed to deal with his crap upbringing , so to me it felt selfish because DC and I came second to the easy option.
I got completely that his relationship with his mother had screwed his head up but he could have faced it , dealt with it. He kept doing the same thing, he would go No contact and then he would let her back in. Several times it happened at moments where I was at my most vulnerable and he basically expected me to be the emotional punching bag so he didn't have to and could live out his fantasy of having a decent mother.
But here's the thing. If , even at the last moment, hell even after the separation he had put me first. Put our family first. Accepted that his need for maternal validation wasn't more important that the health and emotional safety of his family.....I'd have forgiven him. We would have stayed married. All I needed from him was to not take the easy way out and be with us. Not to put his need for her toxic relationship above us, I would have stayed. He would have had to show me that he was finished being selfish and prepared to do the hard work to deal with his issues.
He didn't. Unfortunately it took the loss of us for him to do so. Its taken years but he finally dealt with his issue with her and stopped seeking her validation. But...he did it too late and he knows that. He regrets it but its been years now and whilst we co parent well , we have a great friendship I have moved on.
The reason I'm saying this is I was sat in your husbands place, and I would still have forgiven after everything. But not if he still put that relationship ahead of his family.
You still have the ability to save it (probably) I genuinely wouldn't want you to deal with the same regrets he does.
I'm afraid I did say the same to him. I didn't mean I was better than him I meant he wasn't doing anything to keep us, that he put someone else ahead of us everytime things were settled.
Now you may be in an entirely different place, he may be abusive and unfair. But on the face of it I've been on the other side where I was not the abuser but my exdh put me in the position of having to deal with one.