I usually talk to my horse for a minute tell her to calm down take a deep breath then I am ready it has worked every time so far!
I had a lot of troubles with nervousness at the beginning.
The problem with liberty shows is that there is no link. And at the beginning I was so stressed that it didn't work with my horses, they didn't have any landmark anymore.
I had to work on it, and tell myself 'it's just like at home', I relativise.
You have to know that your horse needs to trust you, you can't give your nervousness to them.
What will help you is to have a good preparation: go at the show earlier, see the ring...