If I might add just a couple of things.

Something you might be doing is comparing yourself to him. In my case I tend to do it almost consciously in relation to my wife's ex-boyfriends. Other people tend to do it more unconsciously. If this is the case, consciously try to address these feelings or thoughts and remind yourself that the past doesn't exist anymore. Only the present matters.

The second and more pressing issue may be the matter of the past encroaching on the present (which is my problem). You didn't say if that person from the past keeps contacting her. That can really exast ... exaserb ... aggravate the situation. On a personal note, I don't contact wives/girlfriends of my friends out of respect for them ... whether or not I'm friends with their loved ones. Even if there is nothing going on, it can really, really give that image.

So yeah, that's just my opinion. Try to avoid comparing yourself to others if that's happening. Best Wishes.