I'm not sure from who or from where I heard this from but the best piece of advice I've heard is to just write everything out, and not take time to stop and go back. I think of it as "smashing through it" because a fist that smashes through something incredibly hard isn't going to stop and think about what its fingers are doing; it's going to proceed straight through the brick wall. This piece of advice has been really helpful because it lets me get down every thought I have down on paper (or a computer!) first while preventing my self-doubt from taking hold in the back of my mind. It allows me to have a basic rough draft completed before I go back and revise and edit my work.