I wish I understood sooner how helpful books can be in learning about history, and there are so many good books to read about history that are so engaging and really just give you a really broad perspective and appreciation for the people in the past.
Something I wish I had understood sooner is like now, very few people are all evil or all good. A lot of times we look back in history and we expect either our heroes or people we look up to or even those that have been considered villains throughout history to be entirely one way, either entirely evil or entirely good. But the history and reality is a bit more nuanced. So I think if I had understood that a little sooner, I would be able to approach historical understands with a little more realism.