Hi rish_dutton,
You're not alone. Many students have no trouble successfully answering practice questions, and completing practice tests at home, but on test day, their test anxiety significantly reduces their performance.
Some possible strategies to reduce test anxiety include exposure therapy (visualizing exam day situations that trigger your fear response), positive visualization, reducing negative self-talk, and turning anxiety into excitement.
This article has several suggestions to reduce your anxiety:
How to Eliminate GMAT Test-Day AnxietyIt's also worth mentioning that overcoming obstacles such as test anxiety is all part of the test. What's important is to develop the mindset necessary to gain insights from your weaknesses.
Here’s an article that explains this in greater detail:
How GMAT Students With a Growth Mindset See Their Mistakes