Your inner critic is a great editor and a terrible creator. Don’t let it in until the draft is done.

Criticism inside your own head is excellent at one job and hopeless at the other. It spots a weak line instantly, which is what you need at the end and ruinous at the start, because nothing survives inspection while it is still forming. Letting it in early means you polish sentences that were never going to be in the final version anyway.

Split the two jobs into separate sessions. Draft fast and badly, and refuse to fix anything until the whole shape exists. Then hand it to the critic and let it be as harsh as it wants.