The burned hand teaches best.
A mistake done is a chance to learn; the greater the pain the longer the memory. Make sure that all lessons dearly paid for are being properly evaluated, and that the refined conclusions are circulated and made available within your sphere of interest.
Quote ― The Two Towers
But if I had spoken sooner, it would not have lessened your desire, or made it easier to resist. On the contrary! No, the burned hand teaches best. After that advice about fire goes to the heart.