This was my experience of the whole thing.
When I found out, I couldn't really accept it had really happened because I wasn't ready to. So I was 100% in for R because I could not deal in any way with the idea the relationship was over. It is an enormous thing to deal with - and like you describe, how the heck are you supposed to imagine your life completely differently overnight? You just can't.
My WS was like yours and did everything perfectly for the first three months, and then he broke NC with the AP several times. Tricky circumstances as they still worked together, but basically he blew it. So even though your WS is doing everything right now - those first few months are panic mode for him too - you have to let things brew a bit to see what he does.
The real decision to R was something I made pretty gradually and there are a few reasons I decided to do it:
1. I still really loved him and he really loved me and I felt sure of both those things
2. Our relationship was still good - very loving, supportive and he was a plus instead of a negative when I was in pain.
3. In my relationship before him, my fiancé had died very suddenly and I didn't want to experience more loss
4. I came here and read the success stories and felt buoyed by what I read and I genuinely believed we could do the R work and come out of his better than we started
5. I didn't think the A was an "us" problem, but more of a "him" problem and he recognised that from day one and committed to resolving it.
6. I fundamentally did not believe on any level the A would have happened if we had not been long distance at the time.
7. My friends and family supported the R and nobody thought I should throw away the relationship.
8. This is the worst one but let me be honest - I was worried he would end up with the AP if I didn't (raw confession there but it's true, she was chasing after him and I figured he'd run for comfort)
9. I ultimately believed he was a good person who genuinely felt immediate remorse
10. He handed over all their communications and I saw evidence the A was more or less FWB with no romance or infatuation on his part.
11. I realised I was going to be in pain and have to suffer regardless, so whether we were together or not nothing was going to save me from that.
12. I ultimately didn't want to spend my life without him
So they were the reasons I stayed and fought for things. Ultimately for various reasons he didn't end up living up to the changes I needed to see (he struggled with the demands of R and tried to rugsweep) and we are separated now but I don't regret the choice I made because I made it for all the right reasons.
What I can tell you for sure is that R is a decision you don't make once - you have to keep making it over and over again and how you feel now will change a lot over time. I thought nothing could be worse than DDay, but over time the pain it brings, the trauma, is so much bigger than you think at the start. It's very tough. Likewise he will change over time and you will need him to "keep it up".
As for how you live with it - you have to live with it anyway. That is the proverbial shit sandwich. You find ways to process it, and then you live with something you can't live with. Because you have no choice. It does get better and relationships are kind of fluid. The old makes way for the new and he and you can rewrite things now.