I read a quote somewhere "remember in the dark, what you saw in the light" - what I do is list the reasons why I believe in a plan or something, and when the doubts come, I validate it against the list of reasons. If the doubts actually invalidate the reason, it is a valid case for concern, but more often than not, its just me forgetting the reasons I first believed in something. This works when I start a project too - a list of reasons why I need to complete that..