Why does my best friend hate me?

So, I have this REALLY REALLLLLLY good friend, Mara, and I have a very good feeling that she hates me. She is SOOOO untrustworthy and tells EVERYONE my secrets. She also talks mad **** about me behind my back. She called me fat and ugly yesterday to my friend Benny!! Stu. But shes soooo pretty and popular and I really dont want to loose such a good friend over such stupid reasons. What do I do :(