Which is the worst misjudgement?

[Edwardo Piet]

In the short run, denial of problems is definitely more fun than seeing problems that aren't even there.

I'd say in the long run, sometimes excessive pessimism is worse, sometimes excessive optimism is worse: because the important thing is that we stay in reality.

In the short run, excessive optimism is definitely preferable. But that's part of the problem -- it can make procrastination more attractive and impulsiveness and addiction harder to resist and avoid.