My bff has been aggresive to me whenever i get close to having a r-ship....why?

everytime i get involved with some1 my best friend is constantly rude to me. she likes to go out and i dont as much, but when i dont have a bf its not an issue, but when i do she blames the fact that i have one as to why i dont go out. she also says im not there for her but she doesnt even call me when she needs me. i just feel like she brings me down every time i hang with her bc she has all these vanity issues. i care about her, bc she has been there for me in the past (friends since 10, now 22), but for the last year and a half she has just been aggresive and mean to me. she picks on me and belittles me and the choices i make in my life and it really hurts me. i was supposed to meet up with her and another friend the other night after i went on a date with my bf and she fought with me bc i wasnt thrilled about where i was supposed to meet her and started calling me a bad friend...regardless i tried meeting up with them and she ditched me and wont answer my calls, texts, or ims. wtf?