The original thought was for it to replace “It was fear that kept me here,” to avoid retconning Luke’s arc in the last film. So it would go:
“I was was wrong (to stay here.) [pause] Lesson three: What are you most afraid of?”
So, it’s like the title “Lesson three” opens the entire topic of “facing your fears,” rather than just buttoning up the final moral of the lecture.
This could also work.