I had the same problem, listen to music or think how nice Horse riding is!
I would suggest going on a short walk and just taking a moment to lead your horse round and get a few bonding moments if you always get nervous,maybe consider getting some classes in it.
I would take a walk and just breathe, another thing I like to do is talk to myself when I’m nervous. If I had a bad fall or a bad ride earlier I make sure I am prepared and get in a good mentality. Finally I would suggest spending time with your horse, grooming,
Take long deep breaths and tell yourself, “I can do this”
For me music gets me in the zone. For nerves i would also have some water and keep moving around. Don’t sit. Also try getting your own horse ready (if you don’t)