Swerving an arbitrary amount in the right direction will not do -- reversed stupidity etc.

Reversing stupidity is not the same thing as swerving an arbitrary amount in the right direction. And the amount is not arbitrary: like most of my belief changes, it is based on my intuition. This post by Robin Hanson springs to mind; see the last sentence before the edit.

Anyway, some positive thoughts I have about myself are obviously unwarranted. I'm currently in the habit of immediately doubting spontaneous positive thoughts (because of what I've read about overconfidence), but I'm beginning to suspect that my habit is self-destructive.

If someone finds it hard to do what they believe that they should and can, then their belief is mistaken, or at least incomplete.

Well yes, of course, it's easier to do something if you believe you can. That's what I'm talking about--confidence (i.e. believing you can do something) is valuable. If there's no chance of the thing going wrong, then you're often best off being overconfident to attain this benefit. That's pretty much my point right there.

As for your Heinlein quote, I find it completely unrealistic. Either I am vastly overestimating myself as one of Heinlein's elite, I am a terrible judge of people because I put so many of them into his elite, or Heinlein is wrong. I find it ironic, however, that someone who read the quote would probably be pushed towards the state of mind I am advocating: I'm pretty sure 95% of those who read it put themselves somewhere in the upper echelons, and once in the upper echelons, they are free to estimate their ability highly and succeed as a result.