People do grow apart. People change due to their respective life chances/experiences....it's sad i guess, but the reality is, she has changed, you have changed...she's not the type of person she once was, you're not the person you were, you may well have a new best friend/set of friends, she has too...
Life progresses, it's only logical such things like this occur - life is not stationary, which is arguably a good, healthy, normal thing/progression.