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).
Example sentence:
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) _______.
Best Answer
1. I've been through a lot in my life and turned me into a fighter.
2. The experience determined the very things that didn't kill me but made me stronger.
There are connotations here of ORDEAL, TRIAL, TESTING, and of a process of distillation, in the sense of separating or extracting the essential and vital elements within, and of a refining process, in the sense of being purified of what is coarse, debasing, or superfluous, again it is a reduction to what is essential, vital, true and strong.
This is the purpose of the alchemist's retort, and of the crucible.
3. These ordeals were my crucible."