@heartlex I'm not saying you have to say anything, I'm just worried for you that you are in that trauma bonding phase. I know right now you would rather feel angry than guilty for not trying, but that will change.
I also think you sound really lovely and he really doesn't deserve your commitment in this, but you have chosen to stay and thats totally up to you, we are just trying to prepare you for what COULD happen. Just because she may be gone, it doesn't mean there won't be anyone else.
Even me, I look at things like this and think how on earth can you stay with someone who cheated on you, that is madness to think that another person can step out of your relationship whenever they feel like it and then get all upset when caught, and the innocent party is left with all the feelings of hurt and decisions on what to do, and really we never made the choice in the first place.
BUT I did it, I stayed and ot still confuses me, and I was only saying to you how I felt afterwards, I felt all the same things as you in the moment. It felt like it needed an immediate decision and I couldn't process it quickly enough so I stayed because I loved him. Over time my feelings were changing.
Its a weird thing isn't it love? How any one person can give is this much happiness or this much sadness. Its amazing how our brains work, if this was anyone else there would be no issue in cutting them off, but as soon as love is involved its very hard.
Sorry that bit wasn't relevant and not helpful, but its just something that made me think.