Are there many confident relaxed license exam test-takers out there? Maybe, but they aren't the ones I hear from. "I blanked out after the first how half-hour" or "I came to a question I didn't know and my confidence collapsed" are typical.
From my former life as a therapist I know it is useless to reassure, to tell someone, "Look how hard you've worked to prepare" or "You have been doing so well on the practice tests, relax!". Nice tries, don't help much.
I don't have the power to calm your nerves, but you do.
First (and foremost) notice it! What specifically happens to you when you start to feel dread, fear and even panic about taking the license exam? Do you lose your focus, can't concentrate, feel jumpy, can't remember facts, don't trust your judgment? Next, apply the antidote: yes, here are #3 things to do.
1 - To restore your attention/focus, stop, close your eyes, slow your breathing and do a mini-meditation for 2 minutes. (Practice this before the test so you are prepared.) There are many brief meditations available free online.
2 - Monitor your negative self-messages. Notice each one and delete. Replace with "One question at a time, I can do this."
3 - Take a break. Don't have time for a real one? Take a "mini-break" in your seat. Stand up, massage your arms and legs, shoulders and neck. Take a drink, eat an apple. This will break the cycle of tension and allow you to reboot and regain a more even stance.
The idea is for you to regain control of your thoughts and physical state. Try this when you are studying and you will become good at it. Go to the test knowing that you have several helpful tools to use that are permitted in the exam room. You won't feel (or be) so defenseless!