You don't need advice on how to control nerves...you said it yourself: you lack self confidence, but once you're in the zone, you play as you should.
Therefore, self confidence is your issue, not nerves. Since you've already proven to yourself before that nerves don't hamper your playing ability, then that knowledge alone should help you build up your confidence for the next game once you understand that this problem you have is all in your head & is only as big of an issue as you want to make it.
I think Teschy said it best: "Don't give a crap & you'll feel better"
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Mephistoles
If it really is a nerves/anxiety problem, something simple you might be able to do until the game is just slow breathing techniques. Just breathe in deeply and slowly through your nose, hold it for a second, and then breathe out slowly through your mouth. Just imagine the nervousness melt away when you exhale. Remember not to rush doing this.
I've done it before big exams when I was nervous. It helped me calm down a lot. It's simple and easy to do.
It helps to be just a little bit nervous if you're about to go out and perform - it sharpens the senses. If you're too relaxed you won't probably perform very well.
If you're feeling just way too nervous, which I doubt as you write that you perform quite well when you're in the midst of it, try to think about something else, what you're going to do after the game or something totally unrelated to the game. But don't forget to put your concentration back on the game at hand a little while before you're about to begin. A little nerve helps you to perform.
My characters on Disco: [LN]-Andy.Doggett, The.Flying.Betty, Russ.Dalton, Harald.Treutiger