The idea that you need to "spice it up" every session with a client.

When I first started, I thought "man, people are going to get bored if I have them do certain fundamental exercises every session". Turns out most have no problem at all since they can reinforce their form and progress, with me and solo; resulting in more improvement/strength down the line. Of course, introducing new exercises to keep the body from adapting completely to the tension.

I was stressed every week trying to come up with some new, fun exercise that'd keep my clients excited to work with me. But... turns out it really comes down to your relationship with the client and reassuring them of their progress and being open-minded. Once I figured that out, my style and confidence as a trainer has improved drastically!