Absolutistic thinking is effectively laying down laws to a universe that is not going to hear them. This leads to excessive pressure, and inevitably disappointment and frustration.

Your mind needs to go to the exam in order to plan, but it does tend to fixate on all the things that might go wrong. If the mind simply imagines it going smoothly it is a lot less likely to keep coming back to those thoughts and when it does the experience will not be stressful.