Let's say a writer is sitting down to write some first draft copy, yet she can't seem to shut down her internal editor. It criticizes and second-guesses practically every word, and after more than half an hour she's only written a couple of paragraphs--far less than her typical output.
This writer can do one of three things:
1) Quit in frustration,
2) Keep at it and beat her head against the wall, or
3) Ask herself: what else is my brain better suited to do right now?
Failed writers will quit early, or blow off their writing session entirely ("I'm just not in the mood to write today"). Soon-to-fail writers will earnestly beat their heads against the wall and spend all day writing and deleting the same paragraph of text.
Successful writers, however, figure out what their brains can give them and adapt to it. They make the most of their writing time by cultivating a discipline of metacognition.
What do I mean by metacognition? In psychological terms, it means thinking about your thinking. In practical terms, it's building a habit of asking your brain from time to time what it's best suited to do.
Therefore, every so often while you're writing, ask yourself these kinds of metacognitive questions:
What kind of writing task is my brain best suited for right now?
What is my brain unable to do right now?
Your answer will tell you exactly what to work on.