In today's tennis world it is easy to fall in love with the ability to hit the ball hard. The racquets, strings, and modern techniques have given almost all players the ability to use pace to win the point.
It is a very good idea to change things up during a match so that your opponent doesn't get used to the speed of your shot. This can be done by adding spin, hitting some lobs and intentional short balls, or just by focusing more on direction rather than speed.
The key is to hit your normal shot most of the time so you can develop some timing and rhythm, and just mix in a different shot here and there. Many players start using the speed of the shot to help them hit the ball with more pace once they get used to your normal ball. Varying the speed, spin or height of your shot will make it difficult for them to time your ball consistently, and will likely lead to more mistakes on their end.
Try to choose the correct ball to change things up-make sure you are not on the dead run or in a very defensive position. In other words, try to use variety at times when you get a ball that you could have hit with a lot of pace.
Remind yourself of this idea and make sure to be decisive once you have identified the correct ball to change things up.