That kind of news is definitely hard to hear. She probably doesn't want to believe it and that's understandable. If she begins hearing it from a number of people she will almost have no choice to believe it. But overall it's not exactly something you need to worry about. You've confessed and she chose not to believe you. There's not really anything else you can do.
WHY did you tell her? Are you trying to break them up? That's not very nice, even if he is cheating. She doesn't believe you because she also believes you are only trying to break them up. The more you try to persuade her, the more she will think you are against her, which perhaps you are. Drop it. In any case, why would you want to be the girlfriend of a guy you KNOW cheats on his girlfriends?
It would have been interesting to know whether or not you knew beforehand that this sleeze had a girlfriend or not, and what your intentions were for telling her. Either way, the answer remains the same. You do nothing at this point.
Nothing, but you need to take more responsibility and determine your own values. Do not be cavalier with your time, your person and your emotions. Deal with better quality people. Read: The 7 Habits of Highly Effective People by Stephen Covey.
You are messy why are you trying to break up their happiness because you miserable if you knew he had a gf you shouldve left him alone and what are you trying to do be with him because if he cheated on her with you what you think he's going to do to you...