Hang out with friends and family. If the guy who she cheated on you with knew you, then beat his ass and post her nude pics/vids.

I was in a situation where some dude I went to school with kept trying to get with my gf (at the time) at their work. He was one of the reasons we broke up, they ended up dating a few times. Good thing he and I weren't friends, I would have beat the ever living shit out of him. Almost did a few times, but was held back by friends, thankfully.

I actually met up with her about 1.5yrs after I broke up with her, since we had a bad breakup. Turns out she basically regrets ever getting involved with the loser and how it affected our friendship. We're on good terms now, but if I ever find out she did mess around with him while with me, that'd be the end of any kind of friendship or being on good terms.

In the end, you'll see that her cheating ass is worth shit and just realize it's better to let the new sucker..er..guy go through the same thing. Later on, you'll meet someone better and who actually is mature enough for a relationship.