I'm so sorry to hear this has happened. I can't even begin to imagine having to go throught something of this nature.

Your wife has violated trust in every respect. IMO, it seems that because she had an emotional affair, (despite of who it was with), she was seeking something that she obviously wasn't receiving in her marriage/relationship with you. And in that respect, (even if she has ended this present thing with this past bf), because it is something she seems to be seeking to satify her emotional needs, I can't help but to wonder if, (or, more appropriately, when), she will inevitably look again, elsewhere.

I'm not one who sees the doom in things, but your wife's behavior seems to tell a tale.
She wants to dismiss this "affair" without any further thought, as though it was an innocent stroll in a park. She isn't showing any remorse, regret or primarily, concern for you and imo, that alone pretty much sums it up. She's looking for something she isn't receiving with you. And shame on her for deceiving you as she has.

I agree with you that if this was limited to just a physical fling, then that would be easier to deal with, (as lust has a way of dictating some of our behaviors that lead to possible irrational thinking which cause some to act upon those urges).

She has placed you into a position where you must focus on YOU and your needs. Keep up with your private therapy sessions. They will get you through this. Take care of you. It's all you can do at this point.
I'm so sorry this has happend.

Shangrala