That is the great issue with doubt. It doesn’t matter whether you doubt yourself in the smallest things. The doubt is like water that falls through the crack in the walls of your soul, and drop by drop, it can eventually drown you. Doubt should only keep you paused momentarily, as you consider your reasons for doing it and then burn those reasons at the stake, along with all its supporters.
If you are going to do the impossible, you cannot doubt it is possible.