It is not one of the things you're going to think it is. It's not the hard work, it's not the horrifying things like I broke my ankle and ran 5 miles on it 2 days later and continued to do so until I destroyed my knee and my other foot as well. It's not the bad memories and the terror of things I may or may not understand why I am afraid of. It's not the sudden discoveries that knock me down over and over. It's not looking at my life and how so much of the bad things in my life are related to abuse. It's not even the things I've lost, relationships, children, probably marriage, etc.
So what is it? Well, turns out the most painful part of my past is hearing people say (and this is surprisingly often) out of the blue "I just don't know how anyone could hurt a child. Especially a baby". And I want to always say I know exactly how, and you don't want to know what I know. It can get worse, because sometimes they proceed to talk about how they think the child in question will turn out, etc. And I have to just smile and play along, because one of the reasons abuse is so bad, so prevalent, is that people naturally protect themselves by believing it happens away from them. I never know what to think or feel in these discussions. And today I realized I truly hate them more than anything else, and it's worse because they are innocent of wrongdoing aside from not mindreading.
I will say I'm glad it is small enough now I can feel that way.