PLEASE answer this question! Long details, but i need answers!!!!?

I'm 12. There's this girl who's been my best friend since we were 3. We promised to always be best friends, go to each other's birthday parties, etc. Recently(for atleast the past 1 1/2 yrs. ), she's been kind of mean. I don't really want to be her best friend. i sort of wanna be friends, just not bffs (i don't want 2 invite her 2 b-day parties). But, I think she'd take it really harshly i told her. She has other friends, and she likes some of them better than me. But she'd still hate me b/c she's self-centered and thinks every1 likes her(especially me) and i think she feels a little like i do -- she probably still wants me as her friend, but she'd see me telling her as i hate her. Also, her family is like my family-friend and her siblings are my siblings' friends. we invite them over 2 swim a lot, which means we still have 2 live with each other no matter what. It's complicated and hard to explain, but using what i told u, plz try 2 help me. thnx