Maybe you're discouraged that this 6 year relationship hasn't led to marriage. You don't want to lose him, but, maybe - at some level - you feel like you don't really have him and never will.

I have a strong suspicion that he's not going anywhere, or he would have already left. He's there to an extent, but he's already not there to an extent.

Putting him aside for a moment, it sounds like you are depressed. Do you have any idea what that's about?