I hope that you are well. Test anxiety is tough. I have had a few students dealing with this issue. A few things that have helped:
1. Build up confidence in your practice.
Have a method. Practice and apply the method so that it is second nature. Force yourself to be SURE about your answers and to have a reason for every answer that you choose.
2. Do not be afraid to disengage from the screen for a moment to stretch your legs, close your eyes for a second, and REFOCUS. It is a fatiguing test and it is so much more fatiguing if you are always on your toes. Be OK relaxing for a second so that you can actively engage with a fresher mind. Remember that the GMAT is a jog and not a sprint.
3. Keep up your momentum. Be OK moving on from questions that are not possible to solve at that moment. Remember that the point of the test is not to get everything correct but to get the best possible expression of your skills.
Once you click confirm: let go of that question. You should feel far fresher and find it easier to stay calm and focused by keeping your forward momentum.
Let me know if you need more advice on this.
"It is a curious property of research activity that after the problem has been solved the solution seems obvious. This is true not only for those who have not previously been acquainted with the problem, but also for those who have worked over it for years." -Dr. Edwin Land
"Learning hurts but enlightenment keeps you company along the way" -HerrGrau
If you found my post useful KUDOS are much appreciated.