Welcome to ultimate challenge.
Almost every horse gets hyper the first few races they attend. They show up in a new place, There are 50-100 other new horses, Somebody shoots a gun and the race is on. Why wouldn't your horse get excited?
After a few races your horse will learn the drill. They learn that it's not like a saturday ride where you ride 10 miles. They learn that when the crowd and excitment is around them, It's a heavy work day. So in time they learn not to waste energy getting excited at the start. But if you only do 2-3 races a year, You will never get them over that excitement level.
In the meantime you need to try and control the situation. With new horses I often ride the opposite direction for a minute or two and let the crowd depart. I don't want to get caught up in trying to keep up with the leaders. Let them go and get a good head start and then follow at a pace you choose. Keep the horses mind busy doing what you ask. Don't just sit there along for the ride. Ask for gives, circle bushes, sidepass, Just keep asking for stuff so the horse has to focus on you and not worry about the crowd.