(idiom) a person who has been ill for a long time eventually becomes familiar with medical knowledge; metaphorically, one becomes an expert in a field through long experience and trial and error

lit. one whose arm has been broken three times becomes a good doctor; fig. a lot of experience makes a person an expert in the field

One learns more from one's failures than from one's successes; literary expression by [[刘禹锡|劉禹錫|liu2 yu3 xi1]] literally meaning that a hundred victories make it hard to evaluate an enemy, while breaking one's arm three times makes one a good doctor