How do I adapt when a lesson falls flat in the middle of class?

You can plan a strong lesson now. What still throws you is the moment it dies in the room, when the plan is clearly not working and you have to change course on the spot. That is the hardest part of teaching, and freezing there does not mean you are bad at it. It means you care that it works. A plan is a starting point, not a script, and the best teachers treat it that way. When a lesson falls flat, the room is giving you information, not judging you. The skill is not never failing; it is noticing early and having a few simple moves ready to shift gears without panic. That comes with practice, and it is learnable. So build yourself two or three fallback moves you can reach for anytime: turn to the class and ask a question, have them talk to a partner, drop back to a simpler example. Knowing you have somewhere to go takes the fear out of the moment. The flat lesson stops being a cliff and becomes just a turn you know how to take.