Courage

Why is it that when you build up both the courage and the motivation to do something important, it isn't possible to do it?
Is it precisely because it's not possible? So it seems easier? I'm not certain of that- in some cases I guess that would account for it, but not always. Sometimes courage just comes when it comes, like a "déclic"- a revelation, and you know something needs to be done. You just can't do it right away.