I think it was The Inner Game of Tennis, by Timothy Gallwey, that first taught me this lesson: when you are preparing for a class, prepare. But when it comes time to stand up in front and start the class, train.
You may not feel as prepared as you would like to be. The conditions may not be as ideal as you would like them to be. But once class starts, you are who you are, and the conditions are what they are. You go for it. You give your current self to the group, just as you are, and you do your best. You train.
It's hard to stop the inner, critical voice that reacts when you make a mistake. It's hard not to wince or get flustered when your lesson plan just plain doesn't work for a given group. But training is as much a performance as a play (except that many of the lines are not scripted), and the show must go on. Adapt, go where it takes you in the moment.
Of course, you can and should review your performance in the classroom. Every time. But you do that afterwards, when the room is empty or you're at yet another chain restaurant having dinner. There have been weeks when I've devoted my hotel time to staying one day ahead of the students, constantly reviewing the day just past and prepping like mad for the day to come.
Disney is a model for me in this regard. In their theme parks, they have "on stage" and "off stage" areas. When you're on stage, in front of paying customers, you're expected to be in character the whole time, even if you're not on duty and just passing from one place to the next. No gossiping about your friends or school, no talk of religion, politics, or sex. That's for off stage, when you're behind the scenes. Some criticize this, saying that it is artificial and inhumane for workers to be unable to be themselves. But I think it's the one key thing that makes going to a Disney park different than other ones. You're immersed in the story, and they're careful to eliminate anything that distracts from that experience.
When you're training, you're on stage too. Play the part the best you know how. Then later, off stage, plan how you're going to make it an even better show next time.