Think about the last time you played a match and got nervous. Now ask yourself this: What were your tactics in that match? If you are like the overwhelming majority of players I've worked with over the years in that situation, you probably didn't have a strong game plan - you were probably just winging it. And so, unsure of what you were doing, you were (drum roll please) . . . unsure of what you were doing. And it is that lack of surety that often leads to nervousness.
So next time you are nervous, ask yourself what your game plan is, and come up with a coherent one. Not only will this give your mind something to think about other than worrying about your upcoming doom, but it will give you the confidence that you know what you are doing. You still have to execute the shots, but it's a lot easier to be confident when you know what you are doing than when you don't. Plus, this confidence allows you to think a bit more clearly and so play even better tactics. In other words:
Good Tactics => Confidence => Good Tactics => Even More Confidence
Let's break this down. Suppose you get nervous in a match. Rather than just tell yourself not to be nervous, think tactically. Ask yourself what tactics you need to do to win. Keep it simple – that is key. Then execute those tactics in a flexible way. (Flexible as in be ready for anything, but use tactics to increase the chances of the rallies going the way you want them.) If you focus on simple tactics and execution, you may find the nervousness melting away as you've given your mind something simple to focus on. This leads to confidence and a clear mind, which leads to clear thinking and continued good tactical play, which leads to even more confidence. It's an upward spiral that leads to playing your best – or, as it's often called, playing in the zone.
When I talk to players after losses, often they'll blame their loss on nervousness. It's only when I question them that the whole truth comes out!