For like the last 7 years since I was a young teen I always find a new best friend do everything with them, become unseperable, my personality will become like theirs and then within 6 months the friendship dwindles down and we stop being friends completely. Like stop talking, lose all touch. 

And its not even something dramatic that ends the friendship. I think I just retreat into my shell and no longer want to really be social with anyone. And then they leave. I've had so many ex best friends its ridiculous. And sad. Every birthday I seem to have a complete new group of friends.