I'm sorry that you find us here. I remember those first few days and it was just as you put it, "excruciating", I never want to be there again. I had gut wrenching emotions that I'd never felt before.
You will find a lot of good advice and comfort here. When you feel like you are going to go totally crazy, writing out what you are experiencing, and getting feedback from others who have been there, can be immensely helpful. I didn't get here until nearly 2 years out. I wish I'd found it sooner, and in fact I did, but I simply didn't trust anyone in the early period and certainly not any online site, when I couldn't even trust my wife who slept in the same bed with me. Over time, and observation, I think I have found this site to be the most trustworthy.
Hope you find this place helpful. I'm 4 years out from DDay in 3 weeks and 13 years out from the affair, and I find it the most difficult thing I've ever faced, by so much that everything else seems laughably insignificant, even my first wife leaving me over religion issues (although that was hard, it was nothing compared to this, probably because I was so close to my wife after 9 years).
It may sound like I am justifying what he did and by no means at all am I, because cheating is never the answer but I can understand why it got to this point.
Yes, your marriage had problems, and you were part of that. You took your issues and did what you did. He took his issues and did what he did, which in his case includes cheating. This almost always seems to involve Family of Origins issues (FOO). However, when you get to the cheating part, there is usually a lot that you don't know about your partner, a lot, that leads to that cheating. Marital problems do not cause cheating. Personal psychological issues lead to cheating.
My wife and I were no exception, I was sex starved, rejected, and had plenty of opportunity to stray, and the privacy when I was working during the day and didn't have the children around, plenty of single and seeking attractive women around, but didn't. My wife, who had the sex drive of an 80 year old, was constantly deferring sex because of noise from the kids and distractions at home, and was a stay at home mom with 4 small children, had an affair that involved full on sex in our house, with someone she hardly knew, with the kids watching TV down the hall, and giving him a blow job in the garage while the kids played in the yard. How could she do that? That was the "rest of the story".
why stop there when you have done all the other stuff
It is a slippery slope, and people progress, breaking one boundary, then another, then another. In my wife's case, that actually began months before she met the OM, with friends, breaking seemingly unlinked boundaries, secret keeping, gossip, putting me down when I wasn't around, telling people that she thought I didn't like her instead of talking to me about it, and suddenly someone arrives on the scene who is willing to listen, tell her how wonderful she is, and commiserate their problems with their spouses.
However, usually, as sincere as the WS may look as they talk to you, they usually are lying their ass off when they first answer the questions. They are in full self protection mode, not thinking of the truth and being remorseful, but being in the mode of "I got caught and need to minimize the damage to me".
How could he do this to you? How could he give up so easily? Why? That is where the FOO and other past issues start to surface when someone works on it, and not just "for show" but really digs into their own issues.
Good luck.