Practicing an instrument and teaching an instrument are pretty similar; both involve listening, assessing, and seeing what can be made better. Whether you are an active performer, a music student, or a teacher, I think it's important to be mindful of how you phrase your suggestions for improvement.
It's easy, especially when you're practicing alone, to start thinking things like "DON'T play that so loud" or "DON'T screw up that 16th-note run again." And yes, you should strive not to make mistakes. However, "DON'T" is a paralyzing word. Even before you get to the suggestion that follows it, "DON'T" creates tension. In taking away the option to do things how you were doing them without providing a useful alternative, it creates a vacuum that is easily filled with insecurity and hesitance.
This is especially damaging in students who are trying to learn an instrument. Music can be such a personal thing that when you tell a student--in an unproductive, "DON'T" way--that they're doing something wrong, it makes them unwilling to take risks in their future practicing and music-making. This produces boring, by-the-book musicians. You only grow as a musician by being willing to dig around in the sandbox and try new things.
So instead of using "DON'T," try providing an alternative. If you have a student who bends their wrist too much when they play, suggest a new way for them to hold their wrist and explain why it's more beneficial for them to do it that way. If you find that you cut off a note too soon in a long phrase when you practice, try practicing that passage in a bunch of different ways instead of just punishing yourself for not getting it right the first time.
Practicing is about inquisitiveness, and hesitance is the death of experimentation. So when you're tempted to use "DON'T," try using "What if you tried it this way?" instead.