I dont know how to let this go, please help...?

(this is kinda long, so im sorry..) I had a best friend of almost six years, and we would do EVERYTHING together. I mean EVERYTHING. I would be there for her no matter what the situation was, and we were like the school sisters. she was my closest friend. the only friend really.. so last year, i had a boyfriend...alex. I wasnt really feeling him, buty the girl...cathy...decided to go out with him after we broke up. I got a boyfriend(who im still with and love more then anything) about a month after that, while she was with my ex, alex. things got weird. Id invite her everywhere with my boyfriend and i and id even offer to invite my ex also, but she wouldnt invite me to parties and stuff shed have. She told me because it was awkward, and she said that she couldnt be friends with me anymore. I got so upset, I felt like I had nothing, the person I trusted most left me, and I was so lonely at school...my boyfriend doesnt go to my school. I became friends with a different group of people, but it didnt feel the same, obne day, i just decided to call her, and we were fine, llike nothing ever ha[[ened. then, when we were unseperable again, this really bad influenced girl who was into really bad things started to eat with us, and i guess cathy thought it would be cool to be in that crowd. id tell her to stop doing the bad stuff (she asked me to keep her away from it) so I tried,, and I finally just told her i was happy if she was, she called me fake, not a good friend, and that she felt bad for me, and a whole bunch of stuff. It was terrible. i couldnt stop crying...and well, i just old her that if she ever needed me that id be here and she didnt respond. I dont know how to deal with this. I miss my bestfriend. Ive acted like i didnt care but I really do, Its really hard, and I feel so betrayed. I get sooo mad when I see if shes talking to one of my friends. I know that kinda weird. I dont know. help!