Sir ML!
This is a valid concern that affects many test takers.
Test anxiety, a subset of performance anxiety, is something that requires mental preparation of a whole different sort, and is often overlooked when we discuss MCAT prep.
Indeed, there is an overwhelming level of expectation placed on test takers, as many view this exam as the key that will open the door to their medical school dreams. I invite you to instead think of it not as a key but as a tool.
Think of your medical school application as a tool chest, and you are hoping to become a craftsman worthy of building the bridge to medical school. The MCAT, your GPA, each extra curricular, each work experience, each personal statement, secondary, and interview are the tools you can wield to build that bridge. Not every single tool is required to build a working bridge, and indeed, not every tool need be the latest-and-greatest version of that tool. For example, if we compare the MCAT to a screwdriver, not every craftsman needs a $500 power screwdriver with 50+ attachments, magnetic holders, and a flashlight. Will having that power screwdriver make certain parts of the job go easier? Absolutely! Can you put in screws with a plain handheld manual screwdriver? Absolutely.
And yes, that manual screwdriver may make it take a little longer. But perhaps you have better ECs or have a great personal story or absolutely nail the interview. Those upgraded tools will speed things up in other places that will make up for your trusty manual screwdriver.
So the MCAT is not the end-all-be-all of your application process. A good medical school will evaluate the entirety of an application. A 520 will no more guarantee an admission offer than a 505 will preclude it. Trust in your preparation, trust in yourself, and trust that you will gain something worthwhile from this experience, no matter your score. And don't forget to breathe.