"schoolteacher changed me" what does Paul D means? What did schoolteacher do to paul D? How?

Paul D had always considered himself a man in his own right. But schoolteacher proved to him that his claim to manhood was not inherent and that it depended upon the will of another. After wearing a bit as an animal would, a portion of Paul D’s identity was shattered. Schoolteacher also shackles Paul D. Still Paul D survives gaining a unique contemplative understanding of white slave-masters like school-teacher hence your above quote. His relationship with Sethe prompts him to try to find a way to reclaim his humanity, and the process of narration that Beloved inspires proves integral to his attempt.