How to win back my bestfriend that I miss like crazy!!!?

me and her told eachother everything she meant the world to me, We were in a group of about 5 super close friends and I mean really close! but everyone knew the two of us were the closest outta everyone, but then one girl in the group whos kinda the leader... told her she had to choose between me and the rest of her friends, this happened seven months back and she chose me, but she had to deal with so much drama and lies and rumors just because she defended me, so she chose them this time. She cried so much but then ust ignored me later. Im excluded out of the group now 3 months later and theyre still talking about me spreading rumors and I really miss her .. Next year is highschool.. well two months of summer now and then highschool... the leader girl wont be there at the same highschool and im hoping I can win bak my bestfriend, how can I do that?