Honestly my nail has cracked like that before and let me tell you the pain is abnormally just really bad. She must be a strong person or maybe part of being insane is getting more pain suppressing chemicals and endorphins
I see her and Sei I'm the boat. There both kids who were abused by their parents and now have serious issues because of it. They both want to get away from it all and find solace in each, but considering how messed up they both are, it wouldn't be surprising if things still go wrong even when it's just the two of them.