My advice to all my riders: Just ride like you know you can.
If you can ride well, you can ride any horse :) If your nervous, just stick your heels down, body tall, and ride through the nerves.
In the end, you'll be fine. I am sure your trainer would not put you on a horse she/he did not think you could ride, let alone have you show it.
So when you get in that showring, just pretend like you are having a regular jumping lesson and ride like you know you can