You answered yourself when you said "your use to be best friend". Fickleness cannot be overcome but getting new friends can.
you just hang back but stand out a little with organising a little get together your friend will think you stand out and you haven't made your relationship with your other friends bad.
Talk to them about it. If they just can't be nice to you and they don't have your back, they're obviously not a true friend. Talk to some of your other friends about it. See if that person it doing that same thing to your other friends. If so, that person is not trustworthy.
You did mention why the two of you separated; over an argument or your friend found other friends. Go over to your friends house or phone (do not text) and if you need to apologize then do so, but if you don't speak from the heart and tell them you miss them as your friend. It is up to your friend to decide if they want you in their life or not. If your friends refuses to see you again then you must move forward in your life and make other friends and eventually you will end up with one or more best friends.
just be her friend! if you 2 are friends long enough and get to know each other she might start to like you back!
Your friends always come back to you when you are not friends anymore because they realize that you are a good friend and they want you in their life.
Listen, You should be able to trust your friend. And you trusted her and she went back and told your other friends. DROP THAT FRIEND , And the others that are making fun of you about it. Because a friend shouldn't really care about that.
Friends should not make a profit from other friends. Sell it back at the same price.
I can't tell you why your friend was talking behind your back but I can tell you that real friends don't talk behind their friend's back. You should let your friend know how you feel about this and that it is an act of betrayal.
talk to her
If you still like your best friend then try to get her back or let her go with your sister if that's what your friend wants. Or mabey your friend could be friends with your sister and best friends with you.
honestly if he left you for your best friend than you dont need to try to get him back because he doesnt respect you at all. and i hope you and that person you called your best friend are no longer friends, friends dont do that to eachother..
usually its because of jelousy. if it wasnt a good friendship then they have a problem with another friend