My boyfriend came back to me and confessed he's still in love with a married girl he had an affair with. Should I let go?

Tagged:

If we all waited until we were completely over the last girl we were with before moving on, nobody would ever have a second relationship.

You can’t control your feelings. You can only control your actions.

So no, if the relationship is working as you’ve described it, (“I'm im love with him and he treats me as if he really loved me…”) then still having feelings for another should not be considered a deal-breaker.