"Get through a draft as quickly as possible. Hard to know the shape of the thing until you have a draft. Literally, when I wrote the last page of my first draft of Lincoln’s Melancholy I thought, Oh, shit, now I get the shape of this. But I had wasted years, literally years, writing and re-writing the first third to first half. The old writer’s rule applies: Have the courage to write badly." -- Josh Shenk (Lincoln's Melancholy)
You'll second-guess yourself every moment of every day as you write your book.
Sooner or later, you'll realize that the mind-chatter doesn't matter. All that counts is the words on the screen. So get those words written. You can fix it all later. :-)