I have struggled at different times with nerves and playing way below par. My problem was getting way amped up, or getting the adrenaline going. while reading "pleasures of small motions" I learned our emotions, or "fight flight" reaction is unconscious, so we have no initial control, but we can learn to deal with it. It was put there in us to deal with those life threatening situations in an instant without thinking about it. Pool is not life threatening (in most cases), but it does put us out of our comfort level. You ever drop a glass, and without thinking about it snag it out of mid air with perfect timing before it hits the ground!...that's part of what I"m saying about the unconscious control. We go into that mode and our big muscle groups are pumped ready to go, and our brain is single minded in survival.....none of which is real good for fine motor skills, and intricate thinking needed to play top notch pool.
So, what do we do?....well, here are a few thoughts from the book.
>exposure to the situation will dull the response in time.
>don't even try to "block it out" it won't just go away. When you sit back and calmly ask yourself what is really going on here, and what do I need to do to relax, you WILL start to settle.
>I suck at paraphrasing, so get the book!

...it really does make good sense.
never let em see you sweat!, Gerry