Just relax. Remember that it's a game, you win some, you lose some.
Everybody prepares a little differently. What works for someone might not work for someone else.
I put my earphones in, and play a rack of 10 ball. I try to get a sense of how the table is playing while I'm shooting, paying attention to the speed of the table, how the cloth is reacting (to draw especially), and how the rails are reacting. At this point I am purely focused on "feel". I don't let anything mechanical even get close to entering my mind. If you start thinking about your stroke, that's when your game starts falling apart imo.
Sometimes when I'm doing this, I'm playing horribly. If that happens, I simply back off. Sit down for a little bit, get a beer or something to eat, take a walk around the block, etc. I don't "try to fix it". After a while when I feel more relaxed, I start the process over again with a clear mind. If I'm still not playing well, I accept it. I tell myself, "I'm not playing my best, but the scorecards don't take pictures. You can still win this. You're just going to have to put in more work. Grind it out. Winning ugly is still winning." Those victories are the most fulfilling.
Hope that helps. It's basically my pre-tournament routine from golf translated over into pool. It worked for my golf game for years, and it's working for me in pool now. But like I said, everyones different.