He was experiencing a lot of self-doubt and probably self-loathing too. He's always felt guilty about the bday party scene, tearing up his family like that and having to feel Edward's pain with his separation from Bella. He blames himself for a lot of things, then add that to seeing Bella cope so well with being a vampire.
Jasper is a tortured soul most of the time. Having to live thru war and death for over a century, and feeling everything his victims felt. If he didn't have such strong character and willpower he probably would have turned really evil. Plus having to live in a cloud of emotions all the time, feeling everyone's bad emotions and doubt and hate, etc. I think Alice is the only thing that keeps him sane and content. Also Bella in the end which is why he wants to be around her all the time.















