I'm never nervous until I am about to go into the arena, because I KNOW I will forget the trail pattern or do one little thing wrong haha.
I'm always scared I will forget something too!!! I've gone to shows and COMPLETELY forgot my horses feed and their grains buckets. Thankfully, I only feed in the evenings, and just give hay in the mornings, so they were okay the night eating my friends feed she let me use.
I have a fear that the horse I'm riding will act up in some way to mess up the class I'm in. It's happened before and my instructor was standing right there (this was last November) and we had to halt for the judge in a W/T/Halt class and I attempted to halt but in front of my instructor and the horse would not stand still as my instructor tells me "the judge is looking right at you!" needless to say, I did not place in that class >.< I was so embarrassed!
I'm also afraid I'm going to forget something, like part of my clothes or something.
I get nervous about remembering courses. A few years ago when I had to remember the simplest course, I couldn't do it and get nervous, so I told my instructor I couldn't do the class and she pulled me out. She then put me in a class where I only had to do crossrails on the outside line!
See I don't really think I get nervous at shows, just a bit nervous for little things, like remembering the simplest course, or fear of leaving something behind (although I never have because I check things 10x before leaving my house!), etc. I get nervous leading up to the show day, but once I'm on the horse's back I feel so relaxed and calm. I think my nerves are more of like "excited and antsy" nerves.
When I start showing dressage/eventing/show hack, I'll totally stress about remembering the test/cross country course/workout. I remember showjumping courses real easy because I remember the order of jumps and the line I'm going to take but for me dressage tests are harder and show hack workouts haven't always gone well for me in the past. And I have NEVER ridden a cross country course in my life. Should I stick with my lovely familiar jumping? Maybe, but I have recently caught the bug for cross country and you can't do jumping and xc without dressage. Plus I'll be doing dressage/show hack (or hunter, depending how she matures) with my baby for 6 months to a year before we start jumping, and another few months+ before we start showing in jumping/eventing. Got to make it worthwhile going to certain shows - some shows only have show hack/hunter, in-hand, and jumping, and Monty has huge scars on his legs so won't do that well in show hack classes.
I also don't like the idea of forgetting stuff. I know people who've got to the city (3 hours away without a trailer) to find that they'd forgotten their horse!