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. Of course, you do.
However, while 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 your demeanor reflects confidence, authenticity, courage!