Hey, I'm sorry to hear about that. The good news is your 18 and you've got plenty of time to bounce back. If it's over, then it's probably not even meant to be. There are plenty of fish in the sea, it's cliche' but true. I was in a three and a half year relationship that ended, and I'm still ok.
We can give you all sorts of sympathy and tell you all sorts of cliched lines, but the honest truth is that you're young (as Sui said) and thus, you've got plenty of life left and that it's not the end of the world. Buck up and realize that assuming this girl really is a whore (I'm doubting it since you sound angry) that chances are, he lost out.
Go out and enjoy yourself, but don't repress how you feel. Find someone, a best friend most likely, and just vent all of this. Don't let it boil in your stomach and mind or it will get toxic.