If a basketball player freaked out every time they missed a shot, there wouldn't be an NBA. The more mistakes you make, the more you've learned. Learn from your mistakes, forget the score, and move on!
I think it's best to try to relax and not focus on the number at the end of the exam. Did you struggle with knowing how to answer the questions, or did you just make a couple careless mistakes by going too quick? I would take a second to breathe and recognize there is a long history of students, including me, who get super nervous as the test gets closer. Then, evaluate what happened and how you can fix those mistakes to improve your score, whether that is being more careful and going slower, or reviewing your weak areas again. I also would look over this post to gauge how ready you are to take the DAT, and don't hesitate to reschedule if you feel you aren't ready. It's better to take the exam when you feel 'ready' than to take it prematurely: http://datbootcamp.com/blog/when-am-i-ready-to-take-the-dat/
Wanna know something? The day before my test I did some practice sections and got a 14 in ochem LOL! Needless to say I was getting lazy and careless. Don't panic; perhaps you were just really tired and not in the mood to study at all haha. I did a new ochem section a few hours after that and got a 22 in that practice. Practice doesn't mean too much. There are too many variables that can influence your performance.