I listened to the 2004 intro to Psych lectures by professor Wolfe at MIT. I thought it interesting that he pointed out that depressives have a more accurate portrayal of reality and how thing really are than "normal" people.
Contrast that with a normal person who believes in themselves. Sometimes they're wrong but they're not affected by (negative) self fulfilling prophecy.
For example, an officer at Big Investment House would credit his own brilliance for making bajillions of dollars, but blame the collapse of his foolish enterprise on Regulators/Sun spots/Everybody ELSE being too goddamn greedy.
EDIT: typo office -> officer
Now maybe I'm too optimistic about what I can do sometimes, but I think you get further that way because sometimes you talk yourself into things that "on paper" should have been beyond you.
Maybe I was more accurate before. But it was because I was skewing the results. Also note that it's easier to think you can't do something and then fail when you could have succeeded than to think you can do something and succeed when you otherwise would not (i.e. it's easier for your own belief to skew results on the negative side than the positive).