This has nothing to do with the other guy. This has everything to do with "your" girl.
You tell her it's him or you. If it's him, you dump her and don't look back. If it's you, you might want to just dump her anyway.
You *
must* maintain respect. Both self-respect and her respect for you. Don't be a doormat. If she thinks she can go overseas and make out with guys and then tell you, then she doesn't respect you. If she then stays with you after this you're doubly screwed, because you'll have to know that she'll have it in her to cheat whenever she pleases.
How old are you again?

5 months != "love of my life."
If, after reading this, you're pissed, but you know I'm right, PM me...