Before you dump her, forgive her, be the best boyfriend ever and be super extra nice to her, make her want you badly, then dump her while she's eating out of the palm of your hands.
While you're with her though treating her well, cheat on her. Talk to other girls, have one to lean towards as soon as you dump your current one.
You will win throughout the entire process. You'll have vengeance and she'll feel like a slutty piece of shit.
Dump her by saying "I wish I was able to continue our relationship, but I just don't feel like I can trust you anymore or at all, and I can't have that in a relationship, sorry."
If you did it right, she'll want you more than ever, if you do not turn back to her at all or even talk to her, she'll want to die. She'll be in your position times two.