Usually you go out with a bucket of balls and practice. You make a mistake, then you grab another ball out of the basket and start another shot or another point. Although practice makes perfect, sometime you forget about the weight or the importance of that shot.
Try this! Just practice or play a set with one ball. Immediately you will see the difference. Every shot becomes more important. When you miss your first serve into the net, you don't pull out the second ball. You have to go retrieve it. Then you start thinking, "Why did I hit that shot in the net?" With just a little of this type of practice you will see the importance of being consistent, not making so many unforced errors and learning control.
Remember; always give your opponent a chance to lose, by making an error. You must get used to your ball going in and your opponent hitting it back. The "Law of Attrition."
Good luck on the courts!