I quite like this phrase. "What doesn't kill you, will strengthen you". This statement is deemed true when we accept, contemplate what goes wrong, learn from the experience then move on. Spirituality is about mind transformation, eliminating bad habituation and develop and enforce the good ones.
When we encounter any difficulties in our lives, I always think it is because we do not know how to handle the situation, that is why it becomes a difficulty, a problem. It is just like when we are in the school, sitting in an exams, some of us is able to answer all the questions with the right answer and some of us do not even manage to answer half of it. However, the question that we are not able to answer, once we study more, put our effort to resolve it, eventually we will be able to answer. What I am trying to say here is, if we put in our effort to overcome our difficulties, to move on, we become stronger and better. But if we keep thinking of our failures and feel pity but not trying to learn from the lesson and move on, we can become weaker.