I think that relationships have "up and down" periods...sometimes you're really close, other times you're not. I also think that soemtimes folks are in your life for a period of time because that is when you need them, and then as life and times and everything else changes, they drift away. It's not necessarily good or bad, it just is what it is. It can be upsetting or hurtful, but it is still life.
Perhaps you need to move this person from your "best friends" list to your "OK friends" list or even to your "acquaintances" list...in other words, you have to 'reframe" the way you thik about her.
I am in the process of doing this with one of my friends. We have known each other for about 32 years, and the last 3-4 years she has gone through some MAJOR lifestyle changes, none of which she really shared with me as one would expect a "best friend" would do. I also had some difficulties during this time period where her support would have been helpful, but alas, she was not there. Maybe it saddens me, but it is also part of the ebb adn flow of life and relationships.