Don't dude. Don't let her back into your life. It doesn't make you stronger to shoulder this kind of hurt and guilt. What would she do if she walked in on you wearing nothing but your boxer shorts and chewed up t-shirt, and some drunk chick on your couch? Do you actually think she would give you the same benefit of the doubt? For some fucked up reason, she thinks it is okay to have a friend over and hang out in her underwear, drunk, with this dude. Does that make sense to you? What do you think is going to happen when someone is half naked, in their own home with someone of the opposite sex? It happened. You saw it. You are now Egyptian, and living by de' Nile. Women are doing this shit for some reason. They are trying to be like their own fucked up version of men, believing that we all cheat on them or something, and it is like they are trying to cut us all of at the pass or something. And when they get caught they want mercy. But if this was you, she would drag your name through the mud, fuck all of your friends, and stand next to the fucked up shit she did and be like "See what I did?" Our society doesn't punish women for doing fucked up shit. You can't beat her up (and you shouldn't) but they know it and they use it to their advantage. She is literally kicking your ass mentally and you are letting her. STOP LETTING HER. You need to peace out of that mess, and forget about her. That is unless you want to walk in on her one night actually fucking someone. Then look out dude, because you might end up in jail forever. Ask yourself this question: Are you going to be that dude that just lets your girl fuck whoever she wants and be like "God damn it babe, you keep doing this!" and being all mad and doing nothing? Or are you going to be that guy who sees this shit, calls her a cunt and throws her ass out and tells everyone what a fucking horrible cheat she is? Be the latter guy, at the least. The best of people just walk and forget the relationship ever happened. Hardly anyone does this these days. They want their fucked up version of revenge of something. Don't be the doormat guy. Be the walking man.