I'm sorry!

I don't feel qualified to offer you advice on this- my marriage is quite young and on shaky ground as it is. But I can offer some support.
I would feel the same way you did. So don't feel like you were out of line or to blame . The way you described it, it seems you handled the situation very well- I'm not sure I would have. "Platonic" relationships between men and women are a touchy thing... I believe there is no such thing for a guy. I feel it is too ingrained in our genes to take certain advantages if they present themselves. We usually won't admit it, and even convince ourselves the chemicals don't exist in our brains. But they are there- some are better at keeping them in check than others. Could your H be having a midlife crisis?
When I was younger, I had a boss who was having a midlife. He would regularly accompany us MUCH younger group of guys to the bars and carry on like he was 22. He was very charismatic and fun. He was the type that could get groups of girls our age to come sit at our table at restaurants and relished in the fact that he could. In retrospect, he was very much like a used car salesman or politician. He had money and would buy drinks and food for the whole group almost nightly. He also developed a relationship with a girl in the office that was way younger than he and separated from his wife of XX years. At first their relationship was platonic, but in no time, the truth was all over the office. I was too young to realize what a jackass he really was and what he was doing to his family. I was just happy getting buzzed on his dime.
I'm fairly certain from what you described, both your H and the woman had an agenda all along- You bet, he is to blame! Don't fault yourself for acknowledging it.
I sincerely hope you can come to terms with it and move on... Don't leave the door open for him!
I can only imagine how bad it hurts... : (