What the research describes
von der Embse and colleagues reviewed thirty years of research covering hundreds of studies and found a consistent negative relationship between test anxiety and performance across age groups and subjects. The relationship is reliable rather than enormous, and it is a correlation, so it does not by itself establish which direction the causation runs — underprepared students reasonably feel more anxious, and anxiety plausibly degrades performance. Both are probably true at once. What the review does establish is that this is a real and measurable phenomenon rather than nerves being over-interpreted, and that it is worth addressing directly.
Where preparation helps and where it does not
Some test anxiety is an accurate signal. If you have not covered the material, the feeling is informative and the remedy is preparation, particularly the kind that lets you verify your own readiness rather than guess at it. Taking a full practice test under something close to real conditions addresses uncertainty specifically, and uncertainty is a large part of what the anxiety is made of. But anxiety that persists in well-prepared students, or that produces physical symptoms, or that appears in the exam room after months of adequate work, is not solved by more revision, and treating it as a preparation problem tends to make it worse by adding hours that do not help.
Where to go from here
This is a grade calculator site, and the honest limit of what it should offer is preparation, structure, and reduced uncertainty. For anxiety itself there are better sources: your institution almost certainly has a counselling or student wellbeing service, and it is the first place to go rather than the last. Our sibling site theballastprinciple.com covers stress and anxiety at proper length and with a method behind it. If anxiety is affecting your sleep, your health, or your ability to attend, that is a reason to talk to a doctor rather than to read another study guide.