Good lucks to you on G-day!!
Well, in order to stop anxiety, try to do what you like most! Because the things you like most can distract yourself from the fear ...for me, the night before G-day, i read my favorite Japanese manga and thought of someone i'm interested in
..remember to take enough sleep ( go to bed earlier or try to make yourself physically tired so that you can fall asleep right away) ..good sleep will be the pedal for you to peak on the test day!
If you turn panic during test, make use of the short break to assure and convince yourself to re-focus.
All what you can do to peak is to overcome your psychological phobia by whichever means ...you know yourself more than anyone so i'm sure you'll figure out some way