Part of the writer's revision process is knowing when to quit. Sometimes your brain decides, "I've had enough and I can't do anymore!" That's not to say that your revision is done, but that at that time you're not able to do any more revisions. It's okay to give yourself permission to take a vacation from pounding your head against the computer screen. Sometimes you need mental rest. Sometimes you need to go physically rest. Sometimes you need more inspiration or more information to be able to revise your work. Sometimes you need a workshop or advice from another person.
That being said, a vacation doesn't mean that you quit forever. I'm not going to quit running just because my knee was uncooperative. I am not going to sit around and whine about it, though. While I'm taking a break from running, I'll be stretching/massaging the muscles, icing the inflammation, and continuing the strength training to take care of my body so that next time I run, my knee won't give out on me.
As a writer and a reviser, you have to do the same things: find writing exercises that help strengthen your weaknesses and/or relax your mind so when you do come back to your project, you're ready to go. Here are a few suggestions:
- Open a dictionary or encyclopedia. Pick two or three words and write a sentence with them. Use that sentence as the beginning of a quick scene or story.
- Go to a magazine or web page with lots of pictures you like (National Geographic, Flickr.com, Popular Science, etc.) and pick out three pictures. One represents a character, one your setting, the last your plot. Write one sentence to describe each.
- Pick one of your secondary characters who hasn't had much of a voice and write a scene from his/her point of view. Is your main character missing something because he/she isn't listening to this secondary character?
- Pick one of your "problem" scenes and write a list of all the smells, sounds, and objects seen and not seen in the room. What is there now that you missed before as a writer? How will your characters react to the stimuli?
- Pull up a different story, do some revisions, and submit it to a contest. Or, at least work on a different project for a bit.
In summary - Revitalize your mind. Revitalize your ideas. Take classes. Do what you need to do to get past the revision block, but don't quit forever. Never give up revising. To me, it is one of the toughest aspects of writing. But that's what makes the final product worth selling and all the hard work satisfying.