When getting ready, allow yourself a lot of time so you are not rushed. Rushing makes you anxious and that will make your horse anxious so just take your time. Also when getting ready you should listen to your favorite music, loosen up dance a little just have fun!
I totally agree with Maddie O. I used to be nervous as well, before jumping and competitions or after a fall, and spending time on the ground with my horse always helped! Tacking him up, giving him a good brush or bath, doing liberty work and leading him around are all things that helped build trust between us. Also don't expect too much from yourself or your horse, I used to have high hopes for our work but my nervousness ruined it. It's good to have high hopes but remember it's okay to take your time and to not succeed at once! Don't forget to enjoy the ride!
I had this problem- I would freeze up and my horse would be confused and it was a mess. I have since changed horses, the one I compete on now gives me all the confidence in the world and it's just me and him and I have so much fun! It could be that this isn't your problem at all, but just had to share just in case it might help
Remember, it's just you and your horse out there and nobody else, it's your floor and you will do great!
You could try meditation methods, breathing exercises are great like what @maddieomeara
mentioned. :) good luck
Affirmations. Over and over again. Listen to them in the car on the way to the barn. I like Affirmation Pod by Josie Ong. Can find it in iTunes.
Maybe try a usual routine before you get on the horse like you always tack up the horse or do the bridle yourself, spending more time around your horse before you get on will increase your connection with it. Try positive self talk, "I can do this", "I'm going to have fun", "I want to do ....(eg cantering or jumping) today". Also maybe try square breathing, breath in for 4 counts, hold for 4, out for 4, wait for 4 over again, this will control your breathing and get you in a calm mindset