I think this self-doubt comes when we do not listen to our brain ...and here is why : when someone ask you:

• Do you want to go to shopping?
  Well, the brain knows the answer, and if you don't have money he will say NO, but then the conscious intervenes, then this self-doubt appears ... you know you cannot go to shopping but yet you want ... so it's a little bit if struggle between your brain and your soul-your desire.

If we listen to our brain we know the answer without doubts.

Greetings!