Previous bad experience - If you had a bad experience at a previous test then each time you try again your brain recalls that experience and doesn't want you to repeat it. So your brain gives you those anxious feelings to discourage you from putting yourself through that ordeal again. We can re-train your brain to generate positive rather than negative feelings.
Your brain has got confused between your vivid imagination and actual memories - The brain can get confused between real and imagined things and so if you imagine over and over, devising all the ways in which the test could go wrong then these "memories" become tightly associated with the test. We can undo this association and use your imagination in a much more useful way, to imagine success. We don't mind the brain mistaking these successful "memories" as real.
Negative beliefs about yourself - Perhaps you have an unhelpful inner belief, such as "I'm not good enough", despite all the evidence to the contrary. We can change this.
Inner conflicts. I once had a client who was wanted to pass their test as it opened up new opportunities workwise for them. However despite being a competent driver he always seemed to "throw the test away" with silly errors. It turned out part of him wanted to pass so he could take the work opportunity, but another part of him was resisting as it would mean spending long periods away from home. In our sessions together we uncovered this inner tension and resolved it.