Apparently, hurting her is what he does best.
He doesn't want to, he doesn't mean to: it just happens, it just happens and he doesn't have any control over it, none whatsoever.
He wishes he could know what to do in order to prevent it, but sadly he doesn't.
He doesn't know how to prevent it.
He doesn't know how to stop himself from unintentionally hurting her.
Shattering her soul to pieces with every single word he says and with every little thing he does is another thing he is extremely good at.