My initial thought was "how seriously committed were the two of you in your relationship" at the time of this other "fling"?
If you were engaged, it would be more of a betrayal than if you were only "committed and exclusive". I hope you can understand what I am saying.
While I understand all too well, more than I am sharing here, your pain....I would think of it as his final determination that *you* are the one he wants over all others. Sometimes before the step that symbolizes the relationship is "forever" it takes that one last look around the room before you walk through the door.
Remember he moved back from 8 hours away to be closer to you.
I say work hard to rebuild your relationship by starting a "new" stage of the relationship. Yes there is history, but look forward
not back.
It isn't easy, it sometimes isn't pretty, and it takes HOURS of sharing and soul baring...but it can be done.
Good luck.