Hmmmm, out top priority should be our own life & our own responsibilities. Who fits in around that is icing on the cake & it can be many or one best friends. It is never healthy emotionally to have someone you EXPECT to have you as their best friend just because you consider them to be yours. Real life just doesn't work like that.
You need to ask yourself why it hurts so much & what happened in your past to make it hurt this bad. Acknowledge the feeling but you need to process why.