Bro, I'm sorry to hear that. So much harder to do this through typing, but "bitchslap", dude wake up! I don't know the whole story aside from this and your other thread that I replied to but I would say this to any friend in your situation.
First off, what you're feeling is natural, this happens after ending a relationship at times. You need to focus though on you. You're what's important and you're what matters. Your ex doesn't matter. What she does with other people don't matter, it's not your concern. Maybe she's a slut, but that's fine, lots of girls are. Some girls go through phases, you can resent her for it, but honestly bro it's not healthy. Seriously, she doesn't matter, what she does doesn't matter. You can treat her like your worst enemy but that honestly won't help you.
There are over 3 billion women in this world. 3 billion!!! Don't get so caught up over just 1! Do you know how many of these women are cute, sexy, funny and just downright sweet and adorable? A lot my friend. Now, these women need one thing, you
It sounds dumb and a little hokey but I'm serious. If you're horny go out there and talk to these women. They're everywhere. Get yourself some nice clothes, don't try to look like everyone else though, express yourself, but nicely. Don't forget shoes. Seriously, buy nice shoes that go along with your outfits. If you can't shop recruit a girlfriend, or a gay guy. The gay guys are a lot better though to be honest. The sooner you find some nice women you'll be say to yourself, "I can't believe that I wasted so much time with 'bitch', breaking up with her was the best thing that ever happened to me." That is true bro, you guys weren't meant to be together, no hard feelings, that's life.
The fact that you're a nice guy, fucking great! Seriously, chicks really dig nice guys. Don't be a weak guy, people get these confused all the time. You can be nice, not weak. They're different things. You can be firm, strong and nice all at the same time. The mix is important. You can't be brave without having fear. You can't be completely confident with being vulnerable at times. Do you kind of get what I'm saying?
I could honestly write a book on this, but I'll let you dwell/question this for the moment. I hope it makes sense, I think/rant a lot faster than I can type and I lose a lot of what I want to say at times. It's my own fault.
Also, one last thing, if your friends are assholes to you, fuck them too. You don't need negative people in your life at all.
Getting off soap box.