My internal editor is always switched on. I suppose that's a hazard of my profession as a copy editor. I edit not only at work but also in the "real world." If a book I'm reading for pleasure is poorly edited, I have to put it down because I'm too distracted by the desire to mark it up with a red pen. I also mentally edit e-mails, billboards, menus. The whole world is one big mess of misused punctuation and poor grammar in need of fixing.
Because my editor never shuts off, I also edit my own writing as I'm writing. I'm starting to see what a detriment that is. I can't continue with my train of thought until I have fixed the commas in the previous sentence. My editor says find a better word, tighten that wording, that sentence sucks and you must drop everything and fix it right now! So I do. Then I wonder why I managed to write only 500 words when I had been aiming for 1,000.
I don't think I can turn my editor off, however. Not after 10 years of constant companionship. Maybe I can ignore him (yes my editor is a him, I don't know why). Or better yet, I can write so fast that I outrun him. I'll give that a try, and maybe I'll finish my current rewrite by the Nov. 7 deadline for my critique group.