Grades are simply a measure of how well you did on one test on one particular day. Some people think that a test score measures what a student accomplished in a course. This is of course false. A test score is a measure of what a student scored on a test. If you are anxious about a test and blank out, then the grade cannot be a measure of what you learned.
Common thoughts are.
If I fail the test , I'm a failure. The truth is that if you do badly on a test you are a person who did badly on a test. Carrying around misconceptions about tests and grades can put undue pressure on your performance. It's like walking on a railroad track. Most people can stay balanced and find it fairly easy to walk 10 feet down the rail.