So your boyfriend breaks up with you and the girl on the side thinks she is the shit because your supposed 2 be man slept with her. How can you curse her out and make her and him feel as bad as they made you feel. Normally not a revengeful/catty person but...
I know right now, given that the situation that you're in, you would probably love to own voo doo dolls of that girl and your ex.but if you were to do anything vengeful, it could hurt you more in the long run. One, it might affect your reputation and your ex, that girl, and some of your peers might view you as the jealous ex-girlfriend.
Be the bigger person and don't sink to their level.
Cursing her out won't help you resolve with your hurt feelings, nor help the situation. If anything, it will only prolong it and create more drama for you to deal with.
It won't make you feel better to pay them back. The fact is, later he'll probably do the same to her and she'll know how it feels. It will happen to him as well. You can just about bank on it, but you have to be patient. And it's not worth the effort it takes to keep up with their sordid affairs. It just shows them that you are still bothered about it. So do as Rocking said.
Hold your head high, dress nicer, find a new guy and whenever you see them, smile and just act like they're completely out of ur life. They'll feel low and that there's something not right, when usually you should be screaming and being angry towards them. The only way you can make them feel bad is by bringing their ego down, once they brought yours (but let that be a secret.)