I forget the specifics and who came up with it, but: pick an amount you haven’t written before. Then, write that much without looking back. If you can set your word processor to only keep your current line visible, even better. Once you’ve written that many paragraphs or pages, do a fast edit. Use a timer so you can’t linger. Then write that much again with the same strategy. After 3-5 times, do a more thorough edit of the whole thing.

It’s not necessarily how you always want to write, but it can help getting over that initial confidence block as an exercise.