Alex sadly it is true that words can cut more deeply than a knife. Unfortunately when this saying was penned life was more about keeping a stiff upper lip in a time where I am assuming that words were not so acid filled.
On a different note. Nobody who genuinely cares about another would say something to hurt them. They would encourage them not tell them that they are a disappointment. If someone is telling someone that they are a disappointment (when they haven't committed a crime obviously) then it speaks volumes about the character of the perpetrator and that they have issues or hidden agenda's of their own. In which case by default you cannot give credence to anything that they say.
All my best wishes Milly x