In all honesty I wholeheartedly believe that what Will couldn’t really get over and forgive was himself, and his choices that led to Mizumono. I think he grieved that mistake so profoundly that it took a permanent piece of his heart. Because at the end of the day it doesn’t matter that he was justified in his actions or whatever else. What matters is that he betrayed himself. And that betrayal led to a lot of destruction and suffering.