a black-out: focus on worries in stead of focus on the exam
If there are negative influences on your test, those are usually the result of worrying. You experience something and don't know how to handle it. As a result you may start having doubts about everything. Normal phenomena such as momentary physical strain and a memory lapse, seem to be insurmountable. Instead of focussing your attention on the exam (your task at that moment), you focus on your personal worries. They disturbe you and interfere with your thinking. In short: the difficulty (strain, not knowing an answer) has become a major problem. Go back to the problem and find a better way of coping with it.
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