A small doubt is too big! When you do something as though it is a done deal magic starts to happen. This does not mean that you should not do your due diligence before embarking on something.
Of course, you do! It does not mean you to be reckless. Of course not!
However, once you make up your mind to proceed, then execution is the name of the game. Whilst executing you want a no doubt attitude. You want to approach your task or goal in hand with a zero doubt attitude.
When you approach a goal as if it is a done deal, suddenly there is a twinkle in your eye and an echo in your voice that reflects confidence, authenticity, courage!