Unfortunately if she is choosing to be with this other person there is nothing you can do nor should do - let them be, let go and move on. She has to realize on her own if there is a problem with him but until then she has moved on and so should you.
get over it
Leave him alone and let him make that choice. If you aren't happy seeing him happy, you must not want him back that bad!
If she was real, she will not become an ex.
You do absolutely nothing about your ex boyfriend considering he has a new girlfriend. If he loved you he would break up with his girlfriend and since he has not then he wants to be with her and you need to accept that. Just because you have broken up with your boyfriend it doesn't mean you can just step back into your ex boyfriend's life.
Simple Answer: You don't.
maybe he wants you back.
I'm sure your ex-boyfriend's unfaithfulness doesn't make you a bad girlfriend -- only him a bad boyfriend. Don't be so hard on yourself. He probably would've done the same thing to any girl.
bad luck
Let him have a threesome with your sister (only if hot)
You guys are not meant to be
No, you should not email your ex girlfriend to try and get her back because she has another boyfriend. You had your chance to make the relationship work out with her and since you didn't then it time for both of you to go your separate ways.
If he has a girlfriend, don't go back. He will do the exact same thing to you, that he will have to do to his current girlfriend. Good Luck!