I beat myself up continually over this issue as well as many other issues that I wish I had handled differently
I've read your larger story in your profile.
Self-blame is part of what many betrayed spouses have to work through. Why didn't I notice sooner? What if I'd confronted differently, would I have gotten more truth sooner? Maybe I could have stopped it from getting so far if only I'd....
I also spent some mental energy just wishing I could go back in time for some sort of a redo.
It's all "magical thinking," and it's a waste of emotional energy.
It took me a while to come to grips with this simple fact: It all happened and nothing can change that it did.
Hot on the heels of that acceptance were these 2 thoughts:
1)He did all of this. At every turn, he chose those decisions and actions. He is someone capable of betraying me at this level and then lying and gaslighting me about it. There were "reasons," sure (his "whys" that he fully owned), but when the going gets tough for him or the right bait is dangled in front of him, he's capable of making decisions without empathy for me that will be super destructive.
2) Can I live with that?
I'm still waiting to see if my H can earn back my trust enough for me to stay.
I want to remind you that you asked your H to come back home, and he refused. He said he thought you needed more time apart. This set-up allowed him to go on those movie dates at her house. He had been treating this "friend" as a girlfriend for a long time already--secret talks and lunches, lots of emotionally-charged interactions. He knew full well he was putting himself in a situation where sex was a likely outcome--her house, alone, him living in his own apartment so you wouldn't know his comings and goings.
When you've wanted to blame her for her Jezebel behavior (and YES it was!), he reminds you that he is also to blame. Believe that.
He made vows to you. (Although she is a broken human being, she made no vows to stay faithful to you.)
He broke those vows over and over.
Forgive yourself. Lay the blame where it should be: on his shoulders. See if he can become a safe partner to you.
(((Hugs to you)))