And she loved a little boy very, very much -even more than she loved herself. The Giving Tree, Shel Silverstein. I want this! Love it. I cried the first time i read this. The fucker cut down the giving tree.
Well there are a couple of possibilities. The first, one can translate it just as you asked: fortitudo e dolore venit "strength comes from/out of pain" But if you want to simplify it, one can just use: fortitudo per dolorem "Strength through pain"