My best friend is friends with someone who is mean to me, what do I do?

My best friend, alex, became friends with this girl mary this year. Mary is really mean to me and always calling me a ***** and a lesbian and ugly and i really like this guy and shes always telling me i will never go out with him because he will never be interested in me. even though she went out with him and he broke up with her. she also is trying to take her away from me. when i was friends with her i would invite her and alex over and whenever i would she would cling on to her and not let me talk to her and hurt me physicly. even on my birthday she wouldnt let me talk to her and alex asked me to go with her to the mall and after i did mary started yelling at me saying why didnt you invite me. and i didnt get why she was yelling at me, alex asked me and i explained that to her and she was still angry at me. then later i had a little bbq and invited them and some other friends and we were watching paranormal activity 2, and i was walking over to the bathroom and she tripped me on purpose and i fell on stairs and a beam and dislocated my knee and got a concussion and she laughed at me so hard and kept calling me a klutz and my mom came in and asked what happened and mary said that i accidently tripped over her leg and i was so angry i just wanted to choke her. i wanted to tell my mom what reakky hapenned but i didnt want to get her involved in this. so just last week alex had a birthday party and she kept calling me names like i mentioned before and also a idiot and a retard. she also made fun of my family and called my brother a weirdo when she knows he has a mental illness. so during alex's party she kept her space from her but when alex's long time friend left early she pounced at her and wouldnt leave her side. i said this to her " hey, alex..." and mary yells at me shut up and she did that everytime i talked to her. she left me crying myself to sleep that night. i told alex this and she laughed in disbelief. what should i do???