The single most important thing to understand about table tennis training is that you are training your subconscious to automatically perform each technique properly. When you do a shot, you don't consciously move your feet into the exact proper position; decide what shot to do and where and how hard; calculate the racket angle; guide your backswing in just the right way; time exactly when to start the forward swing; and contact the ball just the right way. You may be consciously aware of these things going on, but they are guided by a well-trained subconscious, that part of your brain that does things automatically.
A good example of this is tying your shoe. After doing it for years or decades, do you consciously guide each movement? No - your hands just fly about doing what they've done so many times that you no longer need to pay attention. You just watch and are aware of it as it is taking place. As an exercise, try tying your shoes except don't make any move until you've consciously chosen exactly what needs to be done, and do this each step of the way. See what happens?
Similarly, a properly trained table tennis player doesn't consciously guide his shots; he just watches as the subconscious does what it's done so many times before. Of course it's more complicated in table tennis since the ball is moving and spinning, unlike a stationary shoe, which is why it takes a lot of training to get right.
So what should your conscious mind be doing in a rally when training and playing a match? Here are five things it should be doing.
First, the conscious mind guides the training process. You have to decide what type of training the subconscious needs, and then train those skills. It should be analyzing what's going on and figuring out what changes are needed and what type of training needs to be done. But it should only be doing this between points.
Second, the conscious mind should be watching what happens. The subconscious can't see what's going on unless you are watching as well. So focus on what's going on. This primarily means watching the ball, but it also means watching the opponent to see what he does. The key is that you only watch, and let the trained subconscious react. This is the primary - and perhaps only - purpose of the conscious mind during a point. (You should also be listening to the sound of the ball as it hits the rackets and table, which helps with timing and reading the shot.)
Third, the conscious mind should remember the feel of a properly hit shot. (This might be the most important and least applied.) If you know the feel of a good smash or loop, you can essentially ask your subconscious to repeat that feel - and it'll usually oblige, assuming you've trained it well. When you miss a shot or the technique is wrong, the feel should be wrong, and you should focus on remembering what that proper feel should be - and your subconscious will get the message, and try to repeat it. Never focus on the shots that miss or that are wrong or your subconscious will begin to emulate those. (The only exception is if you absolutely cannot get it right, and have to consciously analyze what you are doing wrong - but there the goal is to get it right, and then focus on what is right, i.e. that right "feel.")
Fourth, the conscious mind chooses the basic tactics in a game situation. This should happen between points, never during a rally. This is easy when simply choosing what serve to do. It gets trickier when choosing tactics based on possible situations. For example, if you decide to attack the opponent's wide forehand, you have to wait for the right opportunity to do so. If you decide you need to attack the opponent's forehand, your subconscious will get the message, and when it sees a chance to attack, it'll attack the wide forehand. (Some believe they consciously make these decisions in a rally, but often it is the subconsciously doing what the conscious mind has asked it to do, with the conscious mind simply being aware of it as it happens.) If you regularly think about tactics between points, the subconsciously will get the message and will soon reflexively do these tactics at the proper times. At the advanced levels most tactics are reflexive. When a top player suddenly, say, tactically drops the ball short, he didn't consciously decide to do so; his trained subconscious saw the tactical opportunity and did so.
Five, the conscious minds needs to simply let go, both emotionally and in shot execution. An emotional mind will tend to interfere with shots, which is why professional athletes often look so bland just before and during rallies. A conscious mind that tries to take control will only interfere with the trained subconscious. So during a rally clear the mind and let go, and let the subconscious do what it's been trained to do.
So learn to work with your subconscious. Train it and let it go. In the end, it'll learn to execute, while you, the conscious part that's in charge, mostly just watches during actual play. You should almost feel guilty about this - you just stand around watching while the subconscious pulls off the shots, and guess who gets the credit?