Tapping is the way to go.
For those who are not familiar with it here's a introduction.
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Tapping is a proven method to get rid of anxiety and phobias.
The first exercise can be very well used in field.
You tap your karate chop-area of your hand with your other while saying affirmations. Doing this releases endorphines and will help to calm you down.

(That isn't me btw

)
It's hard to notice and it works well.
Try it out next time you're out and feeling anxiety.
-Jav