I wasn't really surprised at how hard things were for Jasper. I mean, he's been around a long time, and he's seen countless newborns. He's just taken it as a given that they have to be wild and bloodthirsty. Bella's self control makes him look again at what he's believed for decades. And vampires are set in their ways anyway, so any change is... not really difficult for them, but more... rare, I guess. That's not quite right either, but it's closer. Plus, it's also making him question whether he should be expecting more of himself. It's making him think that maybe if he asked more of himself, he'd get more. That's got to be hard stuff to deal with, especially since it says that he's only the weakest link in the family because he believes he is.
Some things exist, whether you believe in them or not. ~Valiant by Holly Black