[...] I don't feel sympathy for Jace because he's just so loathsome and because his past doesn't seem to have had any credible effect on him.

I felt that too, when I read that story about his bird. I mean, I felt sorry for the bird, but when it came to Jace, I just found myself thinking "No, sorry. It's not enough. That is not enough for me to like this guy or even feel the slightest bIt still wouldn't make me sympathize with him, of course.it sorry for him."

Now, if Jace had actually made the whole thing up just to manipulate Clary, that sounds a bit more in character for the person I saw when I read the book. It wouldn't make me any more sympathetic towards him, but there wouldn't be that huge disconnection between his child-self and his current personality.