It sounds like she might have some self esteem issues... I dunno though.
And just because a girl is beautiful, doesn't make her a liar and a cheat, but I know you know that
You both have to sit down and have a long talk. Get everything out on the table and start new. Voice your concerns about her cheating on you. Make sure she knows how hurt you are. Tell her that you're having problems trusting her.
Without trust, you have nothing. Thats my personal feelings on things. Talk to her, and try to see if you can someday build that trust back in her.
Good luck
