Know your stuff and relax
Get this straight, It's one or both of 2 things;
1. Either you know it or you don't. Which is it?
2. You suck at testing probably due to nerves.
Everyone gets nervous before, during and after testing. It's just a mater of what degree and how debilitating it is. If its nerves you need to get that under control because it will only get worse with every test you fail. Don't listen to anyone who tells you test anxiety doesn't cause failure. Thats a bunch of macho BS. I know from my own life, when I'm surrounded in complete chaos and dealing with life and death I'm as calm and a Hindu Cow. You put me in Individual Practicals with evaluators holding clipboards I'm in the depth of hell. But I know people who are the complete opposite. Can't handle their sh*t when they get a flat tire but kick balls on major college exams. Thats life.
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As for #1
if you don't know your stuff I have absolutely NO sympathy for you and hope you continue to fail. Get your act together, learn it and know it. If you are in the field you NEED to know it. Nothing left to say here.
Regarding #2
If you're having test anxiety there are ways to deal with it so don't beat yourself up. Understand we all deal with stressors differently and we all test differently. If you have tried all the testing prep styles and tricks but are still having trouble getting a handle on it, and I know some folks are gonna freak about this, go see your Doctor. Tell him whats up, maybe you need something to temporarily reduce your anxiety like a Beta blocker. It won't effect your thought process just... actually refer to #1, you should know what they do.
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Lastly don't think about how fast/slow you did the test. I blazed through mine, last one in first, one out and passed. Didn't mean squat. What mattered was I had confidence, was prepared and well rested. My buddy from school, first one in, last one out and passed. Didn't mean squat because he was confident, prepared and well rested. You getting the point here?
Knowing your stuff makes you confident.
Prepare in a way that works best for you.
Set the books down and get plenty of rest before you test. This is a big one for most people and personally I don't study for at least 36 hrs before a test. Either you know it or you don't!
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I honestly hope you get it figured out. If this is really what you want to do and you do it for the right reasons I have nothing but respect for you. The world needs people who give more and take less.