I am 48, and my W is 44. We have been married 18 years, and we have two teenagers.
My W's A was in 2006. She got too close to a coworker and the A became physical that spring. She confessed it to me that summer, and I had no idea. It hurt, and I was of course really angry too, but she took responsibility and worked hard to regain my trust. She was also diagnosed with bipolar disorder that summer after crashing from a manic episode. We each had individual counseling, and then when I decided I was really ready to move forward, we went to a marriage counselor. Things got a lot better between us. I was also working on my PhD and dissertation, and I guess all the busyness and stress of life just got us off track again.
I had things I wished were better about the marriage, but I didn't realize she was so unhappy. We still had stressful discussions about sex and affection, but eventually she didn't talk about it anymore. She had pretty much stopped talking to me about "deep" things for awhile by the time she filed.
Her A was terrible, but honestly, it was in the past. In fact, she seemed to have a much harder time forgiving herself than I had forgiving her. She never excused it, but mainly she was lonely, she said she felt invisible, she was very upset about the lack of sex. She would disagree, but I think being manic affected her too.
The hard thing to know is, I am not really supposed to pursue, but it was lack of attention and pursuing on my part that caused her to feel so invisible, so I am not sure how to proceed.