If practice has been done properly, your mind will have stored hundreds upon hundreds of shots. During match play, you simply have to pull the appropriate shot out of your memory banks and apply it to the situation before you. No conscious thought is necessary. All conscious thought should have been done during practice. During a match, your focus should be on the task at hand - moving the cue ball to its desired position and pocketing the object ball. Trust your mind to do its job and pull the proper shot out of the memory bank. This is where the pre-shot routine comes into play.
A pre-shot routine adds to the consistency necessary to perform the repetitive task of bending over, aiming, and shooting. Just as you should want everything during the stroke to be the same from shot to shot, you should want everything before the stroke to be the same. The pre-shot routine gets your mind ready for its task. The pre-shot routine involves looking at the table layout, assessing the situation, pulling the proper shot out of the memory bank, and using the physical aspects of pool to accomplish the goals of pocketing the ball and moving the cue ball to its desired position. Look at the table. Imagine the path from the cue ball to the object ball. Imagine the path of the cue ball after contact. Visualize these things. Allow your mind to pull up the proper shot to accomplish the desired result. During these stages of the pre-shot routine, some players will unconsciously move the cue stick back and forth in their hands. The mind is preparing the body to shoot the shot. Next, align the physical aspects of pool - stance, grip, bridge, and stroke. Address the cue ball by picking out the desired contact point between the cue and cue ball. Focus on the object ball and your aiming line. Do not allow anything into your thoughts besides moving the cue ball to the point of contact. If something doesn't feel right, STAND UP! Proper mental focus requires commitment to the shot. You cannot commit to the shot if something feels wrong. After standing up, restart the pre-shot routine from the beginning. Standing up after bending over takes discipline. Do not allow yourself to shoot a shot with which you're not fully comfortable. Finally, after committing to the shot, stroke it. There is nothing more gratifying than to prepare properly for a task and see it come to fruition.
Volumes have been written about the mental game in many sports. There is a famous one for the sport of tennis. I highly recommend Phil Cappelle's "A Mind for Pool: How to Master the Mental Game." It has 300 pages of information(!) about pool and the mental game. All sports, including pool, require superb physical and mental training. Both are necessary to attain the highest level of achievement. Here's to hoping you've taken the first step to good pool mental health.