Mostly, such situations result from being dumped or from lack of interest to begin with. Normally, loss of love is first followed by a period of mourning, including certain depressive symptoms as you describe them. Your case is different. You had a friendship before you fell in love with him, you weren't really rejected, and you loss is ongoing, rather than clear-cut. Even though you suffered a lot, you never had time to really and deeply experience the loss, and therefore you are unable to mourn and get over it.
Unfed love dies.
Your love for your best friend stays there and grows because you keep feeding it. You might be generally afraid of intimacy. That might be why you fell in love with a guy who was your best friend, whom you knew well, felt comfortable with, and whom you trusted. Maybe that's why you tend to get into impossible relationships, push away potential boyfriends, or date guys who don't really stand a chance with you. What I mean is that possibly, you are afraid to get hurt, and as a defence mechanism, you avoid intimacy without being aware of it.