Well the fact that you are aware that something went wrong, and are able to pinpoint a few mistakes you made is a good sign.
I've only been training a handful of years compared to some guys like JP or Cosgrove, etc. but one thing I've definitely learned is that you have to roll with the punches and adapt to each client individually. Until you really get a solid system for dealing with clients, you're going to come up with something each session that didn't go
quite like you wanted it to.
Other than understanding the body and exercise, the most important thing you need to do is get on the same page as your client. You have to know exactly what they want and they have to know exactly what you are offering. After this first step, you have to get a gauge of what they are capable of, and they should get a good picture of how you work. If you don't bridge those gaps, they'll either struggle for results or just walk away from you.
Imagine trying to learn a single-leg deadlift for the first time, but the whole time you are wondering (a) why my left side was so easy, but on my right I feel like a I'm pulling a circus act, but my trainer said everything is fine... but it doesn't
feel fine! (b) wondering if the trainer really understands what you are looking for, because why the hell am I balancing on one foot if I want to get 'toned'? and (c) I feel like I'm gonna puke because I haven't done anything in over a year and now I've done 200 reps in the first half hour.
Anyway, communication is HUGE. It's a good thing you already noticed it!