AM CONFUSED PLS HELP!!!

 i must compliment you guys for a wonderful job of mending the broken hearts, this program has truly been a source of up-liftment to many.
It is on this note that I write regarding the Challenge am facing.
My name is J.M, There is this girl I met five months ago, she assured me then that she had no one. but I later realized that she was seeing her EX,I confronted her to decided between I and this Ex of hers, else I will quit the relationship, Because I am someone who hates double dating, But she Told me that I shouldn`t bother, that it`s going to end.
The problem now is that ,those guy Craftily tarnished my image before her, showing her only God knows how text messages that are so full of insults, and said I sent them to him.
The story kept me confused because I have never met this guy, I don’t know him, talk more of his phone number. And even I do, of what importance is it for me harassing him?
Another funny thing is that my girl seems to trust this guy cos she looked very serious when she was telling all this to me.
Now what Should I do? I know in me that despite all this, she still have some regards for me but how can I continue with her when this other guy is there? Can I coupe up with his antics? Deep within me I Love her, but, to what extend should I ignore him? That is if I shouldn`t quit, at the moment I have kept a bit distant from her, so help me please!!!