Talk to your best friend ! I feel this way about my best friend all the time! Somebody is always stealing him from me;( at least that's how I feel .. Lol I don't like it and I get jealous! Tell your bestfriend how u feel.. Girl or boy! :)
You steal back your best friend or you can spend more time with him/her and they might realize their strong friendship with you. Another thing, both of you can wear a friendship necklace or bracelet to show this best-friend-stealer that he/she can't steal your best friend.
In short terms these things are gonna happen sadly if it does the only thing you can do is try keep contact with her, but put it this way if she leaves for these other people then to be honest shes no friend, if she really is your friend she will not have to make a decision she wouldnt even think about it she would stick with you forever. Trust me if she ditches you for those other idiots the best thing you can do is either try keep the friendship going or maybe its that time to move on find new friends it always happens