How can I get her back?

I was best mates with this girl for seven years, then this other girl came along last year and completely changed her! I was friends with the other girl but then they both started going off with each other, leaving me and my other friends out, acting as if they were all that and the other girl was being two-faced about her own boyfriend. My friend I’ll call her A was in one of her lessons one day and loads of people in her class told her that the other girl was being twofaced, so we decided to tell her boyfriend, because he is a good friend of ours, and then the other girl was calling us all liers when actually she said it to our faces and she told loads of other people who never usually lie. So this of course caused a massive argument where my best friend went on her side so we were against each other. The next day I got sick of arguments and so did the other girl so we spoke to each other and became alright with each other again, and my best friend said to me she was sorry and that she didn’t hate me etc, so we were alright aswell. But my other friends are still arguing with them which I’m sick and tired of. But I’m mostly upset because my best friend now hangs round with her boyfriend, his friends and the other girl, when I’m with my other friends, which makes another problem. My friend I’ll call her B, is really good friends with one of the girls my best friend hangs round with but my best friend and B hate each other, and now B’s good friend is starting to hate her because of what they’re saying to her. What should I do?? I just want my best friend hanging around with me like old times because we were the friends everyone was like ‘No they’ll never split up’ because we like never have arguments, but now everything’s changed… ??