Part of not being nervous at shows is the rider’s attitude and reason for showing. If you show to win, and that is your only goal, it can be hard not to be nervous. But if you show to have a good time with your horse, and perhaps spend some time with other horsey friend, to measure your progress from & improvement from one show to another, and to generally have a good day with people you like & a horse you love, it is much easier to not be nervous. If you are nervous because other people might not think you are a good enough rider, then you are allowing people who don’t really matter to you to control your actions and make you forget why you like riding to begin with. At the end of the show season, what will you really remember, even years later? The good times you had, or how many ribbons you did or did not win?
P.S. Relaxed riders typically have more relaxed horses, and relaxed horses listen to their rider better and are more likely to be in the ribbons, so not caring about the number of ribbons can actually lead to being awarded more of them.