For example, one of my characters believes the world is meaningless because of what happened to him as a kid but when he learns the truth about the incident he starts to change his point of view of how he sees the world. Should this come at the beginning (the inciting incident) or the climax? Another example is one of my characters has abandonment and trust issues because she thought her family abandoned her when she was young but then learns the truth that they didn't. For her arc, should this come at the beginning or the end? You see what I mean? Should it be what starts their arc or should it be their revelation that finally changes their view on the world?
It really depends on what type of story you're going for? For the first example, are you wanting the character to go through the story showing his hate towards the world for what happened to him as a child, then finally once he learns the truth he can overcome his feelings and move on? If he learns the truth sooner then how would you finish the story? Have you thought about perhaps having him learn of the truth at the end, then possibly writing a sequel where you could then show how he goes about his life after learning the truth? Again with the second example, it's all your personal preference or what you think the reader wants to see. If the truth comes earlier on then your character will be more understanding and not have so much hate towards what had happened, have a better outlook etc. Vice versa if they learn the truth at the end, they'll tend to be more negative throughout.
Ideally you'll want your MC to have some kind of realization just prior to the climax. I call this the "growing some bollocks" moment. Traditionally this is where the lesson is learned and a character becomes complete before they face the antagonist in whichever form it comes. Then again, rules are made to be broken.
You may want to consider two different points of revelation - one where the character finds the truth, and one where the reader does. This can lead to some effective tension. In terms of when the character finds out - my personal preference would be for them to find out late, after they've already overcome almost all of the damage on their own. I mean, letting your worldview be changed by one event, no matter how significant, isn't a great character trait. If you can have your characters realize this on their own and start to recover and get strong through the course of the book, you probably don't even need the reveal about the past issue. (Because it would feel a bit weird, I think, to have two characters both of whom were deeply scarred by a childhood event that didn't actually happen. Not saying it couldn't be done convincingly, but I think it'd be tricky).
