Experience and hardship come to mind but they seem too general and they don't communicate the end result. They can be used depending on the context of course but there might be a word for a type of experience that conveys this proverb better. (if not a single word, a two-word phrase).
I've suffered through a lot of adversity in my life, experiences that didn't kill me, but made me stronger, turned me into a fighter. These ordeals were (my) _______.