my bestfriend

so basically i have known my best friend since 3rd grade. she has always been my bestfriend but we kinda fell off for a year in seventh grade. we are freshman’s in high school. but anyways i was all happy with my boyfriend and me and him suddenly started to fight a lot!! so we broke up and i figured out that he thought i was clingy and annoying. while we were in the relationship i got this weird vibe that my bestfriend and my boyfriend were more fond of eachother than i thought. i thought they liked eachother. so... i asked him if he liked her and he said no and i asked her the same thing and what did she say? no. ofc they would both say no. so i heard a rumor a day or two after we broke up that him and my best friend were talking. so i freaked out and started crying and went to the councilors office and told her everything. I confronted my bestfriend about the situation and she yelled and cussed at me saying that she will never like him nor date him. she lied. she ended up telling me that she liked him and wanted to date him. i said no. like it was obvious that i didn’t want her to be with him. so i said no but like in a polite way. she still said. “well can you tell me yes or no right now”. i said no and walked away. they dated for about less than a month. till she broke up with him the night we went swimming at the rec center. that was friday night. then she told him that she was not wanting to be with him bc he was annoying. she went ahead and texted his best friend and they are dating now. she went from hurting me and dating my ex. to breaking his heart and going for his best friend. what should i do?? be her friend??