Take deep breaths and focus on the horse. Don't focus on your nerves. The more you get your mind off of your nerves, the better you'll feel.
Maybe put in headphones. And hang out with whatever horse u r going to ride.
I'm sure you've heard this before but nerves are a good thing! To keep mine under control, I like to show up early, greet every body, spend a little time with whatever horse I'm riding and relax a little before the lesson starts. It may also help to talk to your trainer about your nerves 😊