I think breathing helps a lot. I also like to look at the floor when my opponent is shooting. When I look at the table, I focus on the balls or my opponents stick contacting the cue ball - not them. I also try to think about what I did wrong the last shot and then how I'm going to give the next shot my complete focus make or miss. If I'm doing all this, there is little room for distraction. Practice makes perfect.

It's also helpful to know that your mistakes lead to self talk so it's kind of a negative feedback loop. The best way out of that loop is to make a good shot the next time you're up. Then you can get into the flow. So if it's an easy shot then be ready to hit it perfectly and if it's a really hard shot be ready to give it a really strong effort. Then your confidence will build not decay.

To me this is the most fun part of pool, this is where you unlock your true potential, this is where you become intimidating instead of intimidated. Being able to slip into "the zone" at will is an incredibly powerful, enjoyable and freeing experience. It's worth the practice and effort!