tbh it may sound odd but try small meditation techniques, they may help calm your nerousness and help you chill before it happens that what i do but i just sit down 10 to 15 minutes before the lesson and just breath deeply and breath out while there is no distraction around you.
you could also do this evertyday for 10 min up till ur lesson
i took something called 'kalms' i think. which literally just helps you stay calm
other advice would be to keep the convo going with the examiner, might make you look more confident
That should be the number 1 tip to be honest.