I think “It was fear that kept me here” is better where it is than replacing it with “lesson three.” Fear did keep Luke there, after all. Fear that the Jedi have done more damage than good and need to die out (at the height of their power the Jedi created Darth Vader), fear of facing those he had failed (admitted as shame in the Ben flashbacks), fear of his own creations (it didn’t scare him enough then, it does now). His belief that his self-exile is a penance and a self-sacrifice for the greater good means it’s not exactly “cowardice,” but it all still comes from a dark place of fear, from a certain point of view.