Whenever I’m nervous I like to think of ten things I can hear, see, and feel around me. For example I was walking up for a cross country course, and my horse had been refusing lately so I wasn’t 100% confident. Walking up the trail these things were really good distractions.
I also like to turn any nerves into excitement and energy for myself when I ride.
Keeping a riding journal is also nice because you reflect on what went wrong/right and why.