I'm going to jump in here for a minute, if you don't mind. Something Al said really jumped out at me.
Quote: That Sheila is much to a wonderful a person to want him, that she has seen that once before and it's only a matter of time until she sees it again. That self fulfilling prophecy is so painful to watch for me. I was there. It ends with the exactly what you don't want.
I realized something about myself going through this. The first time H had EA, it took me totally by surprise and rocked me to the core of what I believed...that he loved me and we would be together forever. When we reconciled the first time, I needed to hear/see things from him to be able to be secure in his love again. Somehow, I never quite got to that point to the same extent, and with the same trust I had before. He says he tried, that he showed me a million times how happy he was to be back together - and sometimes I could see it. Others, I longed for him to say/do/act a certain way, and when he didn't it was like there was this voice saying "see, he REALLY doesn't love you...if he did he would (fill in the blank)".
So for us, it did become a self-fulfilling prophecy. As the honeymoon time faded away and we settled again into just normal life, I didn't realize it, but I was still protecting myself in case he left again. I was keeping a few bricks in the wall, because I never wanted to be unprepared and overwhelmed by that hurt the way I was the first time.
So most likely, I was holding back from H. I didn't realize it, I would have denied it - in my mind, I was totally in my marriage, totally committed. But H says now he felt me holding back or pulling away - that he felt he never quite reached me again after our first EA. His fault or mine? Who knows? Sure there is more he could have done, but he didn't realize it needed done. And by holding a piece of my heart back, I was destroying exactly what I thought I was trying to save.
So the time came when he had convinced himself I had never forgiven him. That wasn't true, but we hadn't healed completely either. So when a cute young twinkie came alone (who as it turns out is very much like me 12 years ago, just without the bad history and with low morals) he was vulnerable. So he took it a step further and convinced himself our marriage was bad, that it was beyond repair.
As Al said, it's difficult to watch, and difficult for me to acknowledge some of it now. But maybe your H is feeling some of what I was feeling. I never would have been able to admit this a year ago, or two years ago, but I know my H was feeling it. I don't even know how much this will help, but I know when Al put it out there, I went "oh, yeah, I did that!"