How to deal with a conceited best friend?

I've been in the same situation before, although I wasn't friends with the girl for quite so long :) As much as you may like to believe otherwise, changing people is EXTREMELY difficult. Rather than focusing on her (what sounds like overwhelming) negative qualities, try to focus on why you DO like her..the things that helped you two to become friends in the first place. It may seem terrible to grow apart from someone you have been close to for so long, but I would advise trying to become better friends with people who you would really enjoy being around, as opposed to "Amy". Try your best to stay friends with her, but don't be afraid to spend more times with others, too. Good luck!!