His best friend died. Killed by Moriarty who John did kill then but, well it doesn't stop the hurt. He's breaking inside and nothing any of us say is getting to him. Its reached the point I don't actually know how to help without just...yelling at him which wouldn't do anyone any good.
I think really he's just existing. Day to day with nothing to live for. He needs to realise what he still has I expect.