My bestfriend is extremely jealous and selfish. HELP!?

i have known her since i was 2 and we are really close but she is really jealous when i do things with this one friend i have and she purposely does stuff to get me mad and acts so casually about it. an even bigger problem is her selfishness. whatever we do it has to be her way and she thinks she is always rightabout stuff when shes not its impossible to win a fight with her and i try to tell her that. she is constantly mean to me.she never just wants to hang out. if i ask her to do something and i know shes not busy and she decides she dosnt like it she will be like "oh ill just do something with (her other friend) instead" and she knows that kind of gets me mad. then 5 minutes later she will call me back and act like nothing happened and want to hang out with me. i try to do stuff to her like she does to me so she could see what it feels like but its so hard to get mad at her. i want to stay her friend but she is really hard to be around sometimes. iknow this is long but please help