Heyo, I don't post often but man, do I resonate with these questions. I'll share what I concluded when I had the same mentality in the wake of my WW's adultery.
Why could have not been like this before his affair?
There's no real answer to this question, which is where the skill of acceptance comes in. Not approval, acceptance.
Of what?
He was not like this before the adultery.
That's it. "What ifs" are great when you are making plans for the future, but when aimed at the past, there is nothing for it but to accept and integrate what was. When it's an unpleasant prospect, like realizing we were living in a lie or being deceived, our brains try to reject integrating it into our life's story. Oftentimes you'll even get the feeling that a new hurt is being inflicted on you. It's not. It's just your brain trying to reject integrating. What worked for me was saying, out loud, "It's not happening right now" and sitting with it. Eventually it gets integrated, slowly, painfully, and then settles in. It's very unpleasant, but entirely necessary. Once you have integrated what is the truth, the pain of it will dull.
Why can’t I be thankful for how wonderfully supportive he is being emotionally and physically without still thinking ‘yeah but he had an affair’!
Because he had an affair.
Your brain will try and reject it over and over and over again while still bringing it up all the time. She does this because this thought clashes with the reality in front of your face right now. As unpleasant and counterintuitive as it seems, try not to flee from these thoughts. These are unpleasant, soul-ripping facts that have to well and truly be integrated into your internal life story before they can be effectively dealt with. There's no reasoning or bargaining with them, they never go away, they never disappear, they just have to be integrated. Once this happens, your relationship to them will change. They always hurt, on some level, but eventually the pain isn't the stabbing, gut-wrenching catastrophe that it is now.
My response to this was "give it all its time on the stage." See how it moves, feel how it hits, notice where it lands, without fighting it or resenting myself for having these thoughts. And oh, how I had them. I remember having sex after the adultery and being suddenly overpowered with the mental image of her adulteror being right where I was, having the time of his life. 8 years on, I don't get the reaction I did then, these thoughts don't even intrude because I have accepted them as a fact, as well as the fact that it will always be wrong that she did this.
My WW would pick up on my pain during these episodes and tearfully ask how she could help me. I told her to figure out how to un-fuck her AP and get back to me. I told her I alone had to pay the interest on the loan she took out. This is really the most isolated you will be in the whole process, because only you can ever deal with this painful process.
I'm sorry you're in it. This is what worked for me, and I hope it's some value to you.
-Mindjob