I have learned that most people are good people and not everyone will hurt you and if they do hurt you it's a reflection on them and not you.
I have learned that I cannot save the world no matter how hard I try and this does not make me a bad person. Making a difference starts with small things e.g. smiling at someone you pass in the street. They could have been having an awful day and you smiling at them might cheer them up :)
I have learned that understanding is the greatest healer and that freedom is in understanding. This requires to let go of a perfectionist attitude and high expectations. Nobody is perfect, including yourself, and beating yourself up about what you should have done etc does not change anything. You get stuck in a rut and trapped in feelings of regret. Trying to understand why something happened allows you to be free, even if the solution is something like "it happened for a reason, I would not have [insert something good or important that came out of it here] if that had not of happened."