Neil Gaiman has said that when asked how to write, his response is:
Put one word after another. Find the right word, put it down.
That is the best answer. When writing, all we are doing is collecting words. If stuck, just focus on putting down the next word. Not a sentence, paragraph, or chapter — just the next word.
Limiting focus allows for forward movement. That is the only way to follow his next piece of advice:
Finish what you’re writing. Whatever you have to do to finish it, finish it.
Doubt lives in the process, but if we let is discourage us from finishing, then the battle has been lost. Finish the work, no matter the quality it is progress.