Im 18 and male, the problem is my ex girlfriend she was the second girl I'd been with but my first love and also happened to be a raving lunatic. She had a bad reputation when I met her but I guess I was blinded by how she made me feel. Anyway long story longer she was a total mind and slept with one of my best friends so I should have no problem with her being out of my life but she is still on my mind constantly and its been over a YEAR! I've had another girlfriend and a couple of 'conquests' since but nobody compares. Should I forgive her and take her back? Or is it a case of 'once a cheater always a cheater' ?
What do I do! Will I ever stop feeling like this?

Look, you said it yourself - 'she was a raving lunatic'. Problem is, she was your first love and that intensity is hard to replicate.

It's not a matter of forgiveness or forgetting - it's a matter of acceptance. You may never forget her (at least until someone as intense takes her place), and you may in time forgive her, but what you must do is accept that she was your first love, that she was bad for you and accept that it's over.

Why would you go back to someone that cheated on you? It just doesn't make sense.

You've already done the hard part of your healing process , why would you want to go back with someone you probably wouldn't trust anyway.

You'll get totally over her with time.

Giggidy giggidy!

No, don't forgive her, she doesn't need your forgiveness and definitely don't even ask to take her back, that's the worst thing you can do.
Yes, once a cheater always a cheater does apply, especially at her age.
Yes, you will stop feeling like this.

Take some tips from Quagmire.

I know how much it can tear a person up when they get cheated on with there best friend or friend in general. One of my ex's had that happen to him all the time. And I slept with his best friend AFTER I broke up with him but he was destroyed and called me saying he hates me and f**k me cursing and screaming and crying saying he hopes I die.. I was shocked his friend told him in the first place. But anyway I love that guy. And he loved me so much he decided to try to be with me again after what I did.. even though we weren't together one thing he always said to all his friends was "don't touch this girl, she's the ONLY girl i've ever loved and i hope you respect that and don't hurt me like that" Anyway over 2 years later were still on and off. So I think give it another chance if she wants to. Don't hold a grudge if you really love this girl try not to lose her forever cause real love is pretty much impossible to find. But your going to have some jealousy and trust problems

Moved from Adult Sexuality because this question is more about the OP's relationship than about sexuality.

No one completely forgets the first love, but we do grow, and move beyond them.