Thanks, I do appreciate the advice
My advisor is...unapproachable. It's a rather sticky situation, to be honest, but it has a lot to do with pompousness, academic posturing and politics, and me not being "allowed" to embarrass him by being in trouble with my thesis...so he's got no idea how stuck I am. I don't know for sure if he'd just orphan me if he knew about my difficulties, but he would most definitely laugh, tell me to "suck it up" and then gossip about me with his other graduate students. He makes fun of other students when we're discussing my work, sometimes, and I really don't want that to happen to me (if it hasn't already) It's sort of just the way he is. I definitely picked the wrong sort of advisor for my personality, but he was the only one interested in the type of work I want to do in my region, so there wasn't much choice, other than to move away, which isn't/wasn't in my best interests either.

There is the possibility of talking with one of the professors on my examining committee, but I'm not sure that they'd agree to not tell my thesis advisor, which is kind of scary.

Hence the stuckness. I don't know if it's just something I have to wait out (maybe I'll get unstuck in due course?) or if there's anything else I can do, other than to keep trying to start small, as serafim suggests. I think that starting small works well for shorter papers (or at least, it has so far...I tend to be okay with the 10-30 page papers for my coursework) but for a 150 page thesis...it seems so huge and overwhelming.