"That's up to you. I can teach you the techniques, but you have to use them." It was mental discipline, not magic. "Are you willing to learn?" Zatanna could have fixed him, the way she'd 'fixed' others at home, but Batman still believed this was best. What was the point if it were merely imposed from outside? The control had to come from inside to matter.
If he'd believed otherwise, perhaps he'd still have Harvey Dent as a friend, but it wouldn't have been real. He had neither time nor patience for pretty illusions.