Being anxious at a show is completely normal... the trick is to prepare yourself for that feeling and when the butterflies start, to have a strategy to deal with it. My strategy is to prepare, prepare, prepare.
You said you got more and more anxious... next time, lunge your boy yourself. One, you'll know it was done, but two, it will keep you busy and burn some of YOUR nervous energy. Then spend plenty of time in the warmup ring, even if its just walking around on a loose rein... make sure you get to the show and arrange for the horse to be in the main arena for several laps so mirrors, flags, banners, murals, etc. aren't a surprise for you or your horse.
Finally, once you go in the ring, think "GAME FACE ON" ... put the smile on your face, get your body position, and think positive. Ride like you are supremely confident in your ability to take those other riders down! :) Stop aiming for first place, just aim for a clean ride, or smooth transitions, or steady posting, - whatever - if those all come into place then the ribbons will follow.
Good luck, and don't worry - we all have had those bad show days, there will be many good ones to follow.
There is no passion to be found playing small - in settling for a life that is less than the one you are capable of living. - Nelson Mandela