Help with understanding....a personality disorder?

I have a friend who I have known for about 25 years. He is miserable all the time. It's taken me 25 years to understand this. He seems very self-absorbed, very self-focused. He doesn't fit all of the criteria for any one personality disorder. I thought Narcissistic...but doesn't fit it all. BPD? Same thing. Depressed maybe, but I think it's so much more. He has serious struggles with relationships. Can't sustain one for long. Very destructive to those he's involved with not because of deliberate abuse, but because he flip-flops so much. One never knows where one stands with him. He has no interest in his family and has basically turned his back on them in a pive way. He is career military. Very unstable emotionally, so tends to shut out emotion. Can someone give me some guidance? I'm trying to point him in a direction, but the map is upside down and turned around!