As KSH said, you need to get out in open water regularly. The best thing to do is to find some swimming buddies to go out with you. Don't be in a hurry. Get comfortable roll around in the water, breast stroke, back stroke whatever. Think like an otter, the water is a playground. Once you're more comfortable, start racing a bit with your friends. When that's comfortable start purposefully running into each other, swimming over each other and basically beating each other up in the water while making some sort of forward progress. If you can do that and stay calm, you should be able to stay calm in the race.

It also helps to swim at the race venue a few times before the big day.

If you don't have swim buddies, rent a sit on top kayak or similar and have a friend go out with you that way. Don't try to water wrestle the kayak tho