The hardest part of being an instructor is...

[Four Bars]

Not saying or doing anything when the student is either not doing something or doing something wrong or at the wrong time... As in when they create their own "learning moment" and they've already been trained for this so let's just sit still and shut up and see if they will eventually recognize and properly remedy the event.
For me, this reaches its  acme while administering a mock PPL or CPL flight test, the "rec ride". It takes a lot of invisible effort to not exhibit any body language and resist the urge to take control and give a tip or demo a simple technique that would drastically improve the exercise. Just keep scribbling debriefing items.
Hard for someone who talks too much at the best of times...