What did Ender learn from the experience with Petra? How did this change ultimately affect him?

Petra is the first of Ender's squadron leaders to "break," and Ender becomes worried because he knows that she has been far from the weakest of his subordinates. Petra's fall teaches him to be wary of the stress on himself, and it teaches him to consider the stamina of his leaders as well as their other characteristics. Ender is no superhero; he says, "Part of what I am is her ... what she made me." Ender knows to beware of his own limitations; his teachers have not been perfect, not even the computer that ran the fantasy game.