With the "nerves" you're describing, you're also feeling your heart racing, hands clammy, and hyperventilating, right? If so, then you're describing a mild case of anxiety attack. This is a normal physiological response when you're under severe stress; biologists call it a "fight or flight response", in which your adrenal glands suddenly release a whopping dose of adrenaline into your system to help you stay and fight, or run for your life! However, too much adrenaline causes the body and mind to be too edgy and can impair how we perform, kinda like after having 4 cans of Coke or coffee! A little bit of adrenaline is good when we train and practice and play, but then too much of it signifcantly impairs the mental game and muscle performance. If you find yourself in that situation, the best thing you can do immediately is to take control of your breathing by slowing it down and breathing deeply, closing your eyes and visualize a serene calm place that you've been to before, and block out your surrounding. As macazteeg has mentioned earlier in this thread (go read it!), then visualize what shots you have to make and imagine that every one of them will be as you intended, perfectly where you want them.