[M/22] got cheated on by ex [F/19], who is now dating the other man.

Knew this girl for a year and a half before we started going out. She was in an abusive relationship (hitting, verbal abuse, possible cheating) with a guy for the entire time I'd known her prior to us dating. I was there for her every time, every fight, every crying spell until she'd had enough.

We started dating about a month or so after she finally broke it off with him, and continued to date for 8 months. One day, she called me out of the blue wanting to break it off. As I was telling a friend what had just happened, he told me he'd heard she had been messing around with some other guy [24/25] for a little over a month. I called her out immediately, she confessed, and that was pretty well that.

Turns out this guy lives in a run-down apartment, has no car, no schooling, has few friends, works as a waiter, and spends most of his time smoking pot and watching TV. Conversely, I'm successful, have a large group of friends who respect me, am fairly good looking (not a GQ model, but you get it), and will be graduating with a degree next year after my internship is up.

And now, less than a month after the break-up, she's started seeing this guy.

Luckily I'm on the other side of the coast until early March doing my internship, and won't have to see either of them until I come back home, but once there, it's inevitable I will. How should I react to all of this? I've been to hell and back with this girl - for this girl - and still care about her despite all the wrong she's done to me. We've grown too close for me to simply shut her out.

TL;DR Girl dumps me, I find out she's been cheating, and we stop talking. I see her in about 3 months. Where do I go from there, if anywhere?