This is a good question, and I'd like to take a moment before answering to thank you for the opportunity to demonstrate that history is the study of many different disciplines, including psychology in this case.

Steps off soap box.

Ok so, the practice of slavery in the United States was older than the country itself. There were slaves in every colony when tea was being dumped into the Boston harbor, mostly household servants and farmhands serving in ones or twos in the North, and the large plantations we are familiar with in the South. Those in the North usually lived in the home with their owner, and worked closely with them in some way or another. When the Constitution was being written, some states, such as New York, chose to end the practice of slavery (Gradually and over time, but the specific process is a more detailed answer than necessary). It wasn't particularly important to their economy so they didn't lose anything by doing it and critically, everyone agreed that it was wrong. Southern opinion on the matter at the time usually called it some variation of "A necessary evil" and focused on how much of their economy it represented. Fast forward a century, and you can clearly see radicalization at work. The prevailing southern opinion on the matter changed from "The necessary evil" to "The peculiar institution". They focused more and more on demeaning black people as subhuman, their right and duty as white people to rule over them and educate them in western ways, and fundamentally the moral righteousness of it all. This clear case of denial was fueled by increasing attacks on something they began to view as central to their identity from northerners they increasingly viewed as outsiders. They began to attach more and more significance to slavery as a part of what they did as southerners, because it gave them a unifying identity to rally around. And also made some massive profit. But the psychological effect is very real, and very relevant to this day. As the two sides drifted further and further into tribalism, the North began to define itself more and more by its opposition to slavery, though not to the same extent as the South did because they lacked the feeling of constant criticism and attack from all sides that pushed people into more cognitive dissonance and makes them double down on their beliefs.