Anxiety, boundaries and expectations?

[canadianMrs.]

Not sure what I should do. I got an e-mail from a friend who works in the same field as H, who said there was a rumour that he had been involved with another woman. She wasn’t sure of the details. Ironically, H was at my house for the weekend. I confronted H and he said he did have a one night stand with a girl he met at a retirement party in December. It happened a few days after we started our 1st MC session, (where C said told H he can't be involved with any one else for this to work). D said he realized after how stupid it was, and never contacted her or returned her attempts to contact him after. He says he never told me because of how stupid of a decision it was and he didn’t want to set us back again. He also said at the time he got caught up in the idea of a one night stand, because he‘d never done that before. I don’t know what to believe. The only saving grace is that this OW lives 6 hours away. He did say though, that he didn’t think it really “counted” as another person in our relationship because it was just the one time. I don’t agree; its one more thing /person I have to come to terms with. Plus, since then, in MC, C said for H to make sure to tell me everything, past to present, because it was my right to make a decision on where I wanted to go with H, based on truth. At the time, all H could come up with was meeting the A OW an extra time during the A then he had told me, but just for coffee. There was 2nd time I asked him to make sure I knew everything that had the potential to damage our M again. He said I knew everything. Obviously not. He says he still wants to work on fixing our M, but I don’t know what to do or think. In the time it took to pick up this girl, and go back to his hotel, where was any though of me, our kids or working on our M? The secrets again.
We did talk for over 6 hours over a 12 hour period about this, and other things. He does seem to be sorry, but I don’t want to be a fool.