Breathing is a sign of the condition of your nerves. As several more people have already said, breathing can be the key to controlling your nerves. This is also something you can practice anytime anyplace. Shallow quick breaths will ramp up your nerves. Deep slow breathing can relax the nerves. Many of us including me can get in the habit of shallow breathing. My lower lungs had gotten in pretty bad shape just from lack of use. I rarely did anything strenuous so I had gotten in the habit of breathing shallow, bad for your health, bad for emotional control.
Sitting here on the computer, watching the other idiot box, practice breathing deep slow breaths, try to fully fill your lungs. This is good for you to do so practicing it for five minutes three or four times a day is good for you. It will also deepen your unconscious breathing pattern. I haven't thought about it but there is a little device the nurse sticks on your finger at the doctor's office. Gives you oxygen satuation or something like that. Less than a hundred bones, was forty when I bought one a few years back. Lets you check oxygen and pulse with no effort. Exercise control over both. Monitor the numbers, chart them, watch them improve.
There is also long term mindset. You came for pleasure. You have planned coming for days or weeks, prepared. Time to execute. Something that is an acquired skill is shutting your verbal brain down, the part that thinks in words. Grab you cue stick and get down on the kitchen table. How long can you go without thinking verbally? Probably a few seconds. Focus on a cue ball or a salt shaker or whatever pretty small, a cube of chalk works and as a training tool if you can't clear thoughts focus on your breathing. Later you can take it to the next level of no thought. This is something anyone can learn. It is an acquired skill, few do it automatically.
Plan your run. Either to and through the money ball or to a lock up safety if it is one shot or two or three. Have your full run clearly in mind then don't think when you are shooting or between shots. As long as your plan is working let it. A major thing is letting an entire inning at the table be one continuous action as long as nothing goes wrong. You planned the entire run. Why think about one, two, or three balls?
Aside from anything else, if you make your time at the table one continuous action it scares hell out of players until they get up to a pretty high level. Two different times in less than a month I had kids come up and want to practice with me. Sure, I was using the only big table in the place and using it at a discount, hard to say no. Each time the kid wanted to finish with a short race. They planned to use me as a whipping boy to finish on a high note. All of a sudden I have quit slouching up to the table and slumping over into my stance. I am gliding around the table with a little spring and impulse in my step. When I shoot I am already moving to my next shooting position before the cue ball stops on many shots. A few games quickly told the kid things weren't going to go their way, they quit midset!
The mental game and clearing the mind are both things that can be practiced many times a day. I loved my jobs in Design Engineering. I could be sitting in my office staring out the window, either working my ass off or daydreaming, both looked the same!
Get the mental game under control and there is no room for the zitters. I was recovering from a surgery and got started in pistol competition to kill time. There was an end of season championship for each division. Everyone that wasn't known started in "C" class and moved up through "B", "A" and into Master if they were good enough. With two months and all scores counting towards your ranking, I had made it to "A" class, sandbagger heaven. One really bad match would crash your average and there were about five sandbaggers in "A" class at the time. One season the top three scores overall in the season championships belonged to "A" class shooters.
Two or three weeks before the championship I decided I was winning "A" class, my division. I didn't say do the best I could or try to win it, I was going to the match intent on winning. I gave it a couple minutes thought six or eight times a day. I was going to shoot well enough to win "A" class. This was my one chance, next season I would be in Master Class. When I got to the match I was "up" slightly, where I needed to be to compete at my best. Far from self conscious zitters. I also was speaking positively, rarely neutrally and never pessimistically. I was there to win. I spent most of my time alone or with people that were unabashedly there to win. Hanging with people that thought they were dead meat wasn't the mindset I wanted to absorb.
While much of this post may seem like a deflection, being prepared mentally and physically is the best cure for the zitters. Even just mentally can be a big deal. For two seasons I shot once a season and won. I did basically no physical prep but the mental prep was strong. The second year in particular was funny. Very few knew me by then and the young guns were comparing each other's scores after every relay. After the match finishing positions were called out, "First place, Hu(gh)" A scramble to the score sheets to verify this! Who the crap was this Hu that had snuck in? I didn't have time for physical prep but I had spent a fair amount of time in mental prep in the weeks before each match.
First you have to win in your mind. Once you have decided to win there is only execution and you aren't thinking when you are executing. Zitters are caused by thoughts. No thoughts, no zitters. Y'all could have read this one short paragraph and saved reading the long ramble. Plan to win, execute, win. Simple enough, short enough to go on a t-shirt!
Hu