*(I write this in response to some dojo'ers that had mentioned to me that they sometimes can't shake doubt. My answer has always been that doubt is 'the lightest form of fear' so don't attempt to shake it, understand it and it vanishes. ) 'from the book, 'Standup Comedy with a side of Chili'.
~Doubting yourself is simply being human. It's just a dream of sorts. Understand that and you won't ever allow it to grow into fear that will make you submit to inaction.
My thinking has always been that if you doubt that usually means you're entering an unknown of some kind ... and if you're entering an unknown of some kind that must mean you're exploring. When you explore not all things are known to you, so you quite naturally doubt a little, it's an automatic way your mind has of reasoning. But that is all that it is.
Exploration means you're on the verge of learning. Doubting simply means you're growing. Doubt is good!
*Also remember that the first to letters of doubt is... do!