I guess I wasn't very clear in what I meant by "second-guessing themselves". I meant second-guessing values and motives, not second-guessing how to act on (or most precisely express) them.
I've mentioned it elsewhere, but I'm just too lazy to find it, so I'll just have to shamefully self-plagiarize.
INTJ: I'm going to do A.
INTJ: Because it's the most efficient way to achieve B.
Me: Why do you want to achieve B?
INTJ: Because it's the best way to achieve C.
Me: Why do you want to achieve C?
INTJ: *blank look*
Me: What's so great about C? You must have a reason for wanting to achieve it, surely?