Why is it so hard to give up friends?

So, Im friends with this girl for 14 yrs (im a girl myself) and we finally aren't friends anymore. I see her everyday at school and it just saddens me how much she impacted my life... in a bad way. I just don't know what to do she's such a selfish person and I still hope her all the best in life, we have a lot of the same friends and of course they all took sides. Some took my side and stood next to me, some took her side and refuse to talk to me anymore. I'm a person who hates drama, im very pive in nature and I just don't know how to get over this its been months but im still loosing other friends over this.