How to get over such a long friendship?

I use to have a best friend, I don't anymore. She has just completely changed and hangs out with a complete ***** now, she has even been saying mean things about me behind my back. I wish I still had a best friend, but it isn't worth being hurt over, so, I'm completely through with her. I just can't help but feel really sad and stressed over loosing my best friend since I was three years old. Words of encouragement and personal stories would help. And no, I don't want to reconcile with her...she's treated me like **** for so long. I just want to be DONE with her, and these sad feelings. Just need to be encouraged :[ thanks <3