Well I will have to admit I felt the same way before EVERY practice exam. It was funny b/c I knew that practice exams were exactly where I wanted to be. I hadn't completed my studying, so the practice exam was the perfect place to mess up (of course I wanted to get 700s on every practice test). But here is the thing, it makes sense that you'd be stressed/afraid to take it. It basically has implications about how you will feel about your studying for the next week or two until you take another.
You have to remember that this is just studying. You have to take it as exactly that. It is the best place to be if you havn't completed your studying yet. Sooo, there's a few things to remember:
1. The GMAT is a rollercoaster ride. Your emotions will go up and down with however you're doing with your studying that week, day, or even hour. Just don't let it get you discouraged, stressed, or afraid b/c it will only get in the way of your studying. Try to take the rollercoaster ride aspect out of it, and be a cold trained killer. Do what you have to do to take the next step towards your goal.
2. The practice exam is your friend. Where else do you want to be? I sure as heck know you don't want to be in the exam center yet. So stop procrastinating and look at the practice exam as your friend. Another opportunity towards your goal.
3. Visualize success. Everytime I was afraid of a practice exam I would visualize finishing the practice exam and seeing the score I wanted, pumping my fist, and being happy the whole rest of the week. Use this visualization as motivation to forge ahead with your class.
4. Take the full length test (with AWA). This will help with your stamina now so you don't have to worry about it on test day.
5. Set mini goals. You don't have to score your goal on every practice test. In fact, you don't have to score your goal on any of them. My goal was a 700, and I didn't hit that until test day. I took 7 practice tests ranging from 570 to my final at 690. Then finaly a 710 on test day. So I set mini goals for each test. It was either a score I wanted to see (in the 600s knowing i hadn't completed my studying) or was a score in a certain section, or really concentrating on my timing or other area to help with the test as a whole.
Goood luck! I've seen you around the forums, and can tell your dedicated. It is normal to be scared or stressed b/c it means a lot to you. It means a lot to everyone. But the true killers out there put those feelings aside and move forward to their goal. If you want to be a GMAT killer, KILL KILL KILL, and do what you need to do to reach your goals.