Wikipedia describes the functions of the prefontal cortex thusly:
The most typical psychological term for functions carried out by the prefrontal cortex area is executive function. Executive function relates to abilities to differentiate among conflicting thoughts, determine good and bad, better and best, same and different, future consequences of current activities, working toward a defined goal, prediction of outcomes, expectation based on actions, and social “control.”
In short; many aspects of the writing process (decision making, delaying gratification, organizing thoughts, working toward a goal) rely on the effective functioning of this small, miraculous lump of tissue. The prefrontal cortex is truly awesome, and it does a lot, but it has it’s limits as well.
If you demand too much of the prefrontal cortex, the circuitry can’t keep up, and a brain fuse may be blown. When you try to think about too many aspects of your writing at the same time (perfect word selection, punctuation, sentence structure, paragraph structure, chapter organization, deadlines, etc) writing might feel excruciating because your brain is straining and can’t keep up. The executive function refuses to function. This is felt subjectively as “overwhelm.”
Overwhelm is a common feature in the blocked writer’s profile. You tend to avoid writing because this uncomfortable feeling is attached to the process. Your prefrontal cortex becomes too full and your brain screams for relief.
A useful approach to consider if this rings true for you is to break down your writing tasks. Resist the urge to solve all problems at the same time. Learn to tolerate the gradual pace of a writing project, and trust that you can fine-tune and polish in successive drafts. No need to do everything at once. Choose a piece of the whole and work on that.
Writing can be a process that brings you your answers as you let yourself engage in it. Let the prefrontal do bits of work at a time, so there is adequate energy and capacity. And when you notice that overwhelm feeling arising, take a deep breath, then pick out one section or problem and just work on that.
Your brain will thank you, and your writing will become more enjoyable.