How do I keep my quarter horse from being nervous when warming up for a show?

I have a quarter horse gelding that does reining, and whenever we warm up in the pen, he gets really nervous, to the point where I just don't trust him so I get off. My trainer gets mad at me for that, but I just don't feel safe, but at the same time, I have to ride him. He's been in the same show pen over twenty times, but each time he acts like he's never step foot in it before. Any tips? I am thirteen, so I'm not the biggest and most respected person in the barn. I'm just scarred right now, but any tips will do. Thanks!