I like your final theory! {Imagine thousands of deleted exclamation points inserted here.}

That sounds very Snape-like to me. He doesn't like to fail at anything. He failed at recognizing voldemort's errors; he fell (I'm sorry, but I have to say it) to the dark side.

Until he can prove that he can work with those forces and not fall and not be defeated, he has this niggling sense of imperfection, like having sand in his shorts (or his shoes). He has to prove himself, not to Dumbledore or to anyone outside, but to himself.