Did you ever read the book The Man Who Lost His Head by Claire Huchet Bishop when you were a kid? If you didn't, the man who lost his head tries on different heads he makes out of various vegetables (like, turnips) and pieces of wood so that he can go looking for his real head. Switching from writing mode to editing mode is a little like the man switching which head went on his shoulders.
Like the turnip head and the wooden head are similar, but ultimately different things, so are writing and editing. When you're editing, there should be no writing during the editing phase, only dismantling.
Fellow horror writer Carrie Green, in her comments for my last post, mentioned that her dream goal "is to be like Stephen King, who maintains a practice of four hours devoted to writing every day. He does, however, put aside work once it is finished before editing."
She went on to say, "Writing involves letting your creativity go free, to explore tangents that occur during the process, etc. Editing is the direct opposite approach. You're cutting away the fat, looking impartially at excessive description and being extremely critical."
When you're writing, you go exploring. Editing is hunting instead of exploring. You're hunting for things to get rid of - typos, excess words and sentences, and the like.
If you've ever had trouble making this transition from dreamer to hunter, even after a break from the writing, try this: pretend you're someone else.
Even better, pretend that you are a very angry person on their most critical day. Look at your work like that person would and start to ax out all of the things that critical person would pick on. Just try not to get too carried away. You want to still have a book at the end, after all.