Most people's friendships are kind of like talk shows. Each year, you have a bunch of people who appear on the set of your life and hang out for a few brief moments. While most guests come and go, there's usually only room for two or three constants, like the TV sidekick or bandleader. While even the best of shows must come to an end, so too do some of the best friendships. It's really no one's fault.
People change over time and so do some friendships.
Just before severing all ties with your long-time friend, make sure that she's not in trouble. Her changing personality could be her way of reaching out for help. You owe it to her to be completely honest. Tell her that she hasn't been acting very much like a best friend. Let her know that your idea of friendship and her idea of friendship are very different.