Doubt is an odd duck because it can become genuinely self-justifying. I know of few other mental qualities that can claim that. If you doubt yourself, then, at the very least, you have the problem of doubting yourself, which is an actual, legitimate problem! Boom: you've just created a real problem out of thin air!
In truth, there is no place for self-doubt. It shows a lack of trust in your past-self and the decisions he made. You made the best decision that you could. (There is an exception, of course. If you made a decision when under the sway of some passing, mental evil: arrogance, greed, malice, hatred, then you might want to revisit the decision.)
The vast majority of mistakes do not come from some personal lack. They come from bad luck. Which is why individuals, as much as investors, need to diversify. Create more than one path to success.
(This is such good advice, maybe one day I'll take it! :)