It sounds a lot like a Borderline trait. My T thinks I am Borderline too, and i go through the exact same emotions as you.
We clutch onto people in our life, rely on them for happiness. I'm trying to deal with these emotions in therapy, and it boils down to working on your own self-worth. Being happy in your own skin. Being happy with your own company. Being able to be alone. To do things that truely make you happy; even if it at first appears to you as being selfish - but you need to look after yourself.

I know it seems like you are 'losing' a best friend, but he's still there. You need to be able to allow him to be happy too - I know it's easier said than done