I think that's one of the hardest things about the GMAT. TB, you need to RELAX!!!! True, it's a huge part of your application but the GMAT isn't make or break you. Essays and other parts are important as well. That being said....
I've found that when I get like you are right now that it's time to take a break. I'm not saying a week or even a day. I usually spend about 4 hours a day studying GMAT and when you keep missing little things here and there it's more because you're not focusing correctly. You've worked your brain into such a frenzy that you're almost second guessing yourself the minute you decide on something. When you feel the wheels spinning, STOP! Take a break. Do something that you find relaxing: play with your dog, watch TV, play video games, go shopping.... I always find that my mistakes go down after I've taken a small break.
woohoo's post has really helped me study better than before. He's got a lot of good hints. His DS system helped me immediately.
I hope that this helps. I'm sure that you'll do very well on the GMAT.
Just remember that the GMAT WANTS you to feel this way. Don't let it get to you like this!