Coming from a guy who just took his test a week ago and feeling EXACTLY the same as all three of you, you NEED to take some time off CATs. i didn't believe all the people who told me to do so 1 week before my test (my last GMATPrep2 score was pretty disappointing, and my confidence was crushed), but I followed their advice anyway. Just the thought of never having to do another practice CAT lifted my spirits!
You won't lose momentum if you do just practice problems (timed if you can) from now on, and keep away from doing questions the day before (go watch a movie, hang out with friends, etc...). You'll find that your hit rate on the practice problems will be good, you'll feel good about yourself, and that positive attitude will get you a good score.
Trust me, if you're getting the same "clouded brain" problem in the middle of a practice CAT, you and I experienced the same thing, and for me, stopping all my practice CATs, avoiding things like GMATClub Challenges
, and relaxing helped clear my brain up and have all the formulas and rules settle in.
Good luck to all three of you, I will hear your great scores next week!