Hi manjot123,
I’m sorry to hear how things went with your GMAT. Given that you’ve spent a ton of time on your GMAT prep and also scored well below the scores from your practice exams, we really need to determine what caused that score drop. Stress could have played a role, but a more likely reason is that in your GMAT preparation, you did not really learn to do what you have to do in order to score high on the actual GMAT. Rather, you picked up on some patterns that were effective in getting you relatively high scores on practice tests.
In my eyes, there is only one path forward, and that is to take a deep dive back into your prep (especially for verbal) so you can individually master each topic, starting with the foundations before moving to more advanced topics. For verbal, you have to learn to see more of what is going on in the questions and answer choices. Doing so will take working topic by topic, carefully answering questions as you work on each topic. You won't lock in a high verbal score by continuing to do what you did before this recent test. You have to adjust your training to become more skilled in clearly defining the differences between incorrect answers and correct answers to verbal questions.
In other words, you have to learn to make your reasons for eliminating choices less like these:
"This choice is too extreme."
"I don't see how this applies."
and more like this one:
"This choice seems to conflict with the premises of the argument but actually fits the constraints established by the premises and fails to attack the connection between the premises and the conclusion."
For quant, since you scored a Q45 (and as high as a Q50) on your practice exams, it’s clear that you have a solid foundation. Thus, to improve your quant skills, you need to go through GMAT quant carefully to find your exact weaknesses, fill gaps in your knowledge, and strengthen your skills. The overall process will be to learn all about how to answer question types with which you currently aren't very comfortable and do dozens of practice questions category by category, basically driving up your score point by point.
Regarding stress when taking the exam, to help manage your nerves, you could try some visualization techniques. Imagine various test-day scenarios such as getting stuck on a question, issues in the testing center, feeling pressed for time, or worrying that you’re underperforming. As you feel your stress responses start to kick in when thinking about these things, practice pulling yourself together and visualize focusing on getting correct answers.
When taking the GMAT, the best way to deal with stress is to get busy answering the question in front of you, focusing completely on the task at hand so that you redirect all of that nervous energy and naturally calm down. In other words, you distract yourself with a goal. Just tell yourself that all that matters is getting the correct answer to the question in front of you, and then when you have finished that question, take the same attitude toward the next one. Focus on winning each “mini-battle” and you won't have time for anxiety. If you can win enough of those battles, you can win the GMAT war, right?
You also may find it helpful to read the following articles:
How to Eliminate GMAT Test-Day Anxiety and
How to Score a 700+ on the GMAT.
Feel free to reach out with further questions.
Good luck!