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.

Are you in the habit of immediately doubting negative thoughts as well? All emotionally-laden spontaneous cognitive content should be suspect.

Also, when you correct an overly positive self-assessment, do you try to describe it as a growth opportunity? This violates no principles of rationality, and seems like it could mitigate the self-destruction. (See fixed vs. growth theories of intelligence.)