I heard about this theory. It sounded so easy, so here it is.
Strategy is important. What is your strategy? Ask a million tennis players and you might get a million different answers. Try this.
There are two types of balls. They are called "Yes" or "No." A "Yes" ball is a ball that you think you might hit a winner with or you will not allow your opponent to control the offense if they get to it. A "No" ball is a ball that you receive and it is very offensive. You would be happy just to get it back, much less win the point with it. Ideally, you want to just the get the "No" ball back so that your opponent cannot keep controlling the offensive.
On occasions, a "No" ball should become a "Yes" ball. Example: Your opponent drives you deep to a corner and advances to the net. If he had stayed back your ball would have been a "No" ball. But, because he attacked you offensively and came to the net, this forces you to handle this as a "Yes" ball. Just getting it back will not work. You must force yourself to try to win the point (passing shot or offensive lob).
The "Killer" is 80% of all balls in a match are "No" balls and 20% of all balls are "Yes" balls.
This might explain why you feel like you hit many good shots, but fail to win the match.
You need to be prepared to handle the 80% of the "No" balls. Everyone will make his or her share of the "Yes" balls. Start saying to yourself, before your opponent's ball crosses the net, whether it is a "Yes" or "No" ball. Do this all the time, especially during your practice sessions. When you get to a match, you will already be trained to compete.
Good luck on the Court!