I would suggest to keep the significance of the event in context. Works like a charm, every time.
Think of the worst case scenario, and by that I mean the worst case scenario and think what would happen if that comes true, then, magically, everything will take its real value.
As an example I can tell you mine. Failing my attempt to get a decent score will only delay me one more year to go to business school. As much as I'm ready and want to do it now, one more year doesn't sound like the worst someone has ever faced in their lifes.
Then I see the benefits of the worst case scenario: I would have an extra year to craft my application, to strengthen any weak areas in my application...
But let's say that my worst case scenario is even worse. My family will stop talking to me, everybody will think I'm a failure and any other silly thing we can think of...
Then I can certainly relax knowing that time will prove me right.
Getting what I want is just a matter of time, not a matter of capacity.
Don't think this is true? Look back into your life and think of all the silly stuff you've done and that at the moment stressed you just like the gmat is stressing you now, and tell me what difference did it make. If you can answer something else than a big fat nothing, then whatever that thing is, must be waaaaay more important than a test. See? It's all about context.
So relax, enjoy seeing how a CAT test is trying to break you and not the other way around.. whatever comes out of it, it will be a positive experience.
Best of luck!
Edited to fix some major typos...