Looking for a children's story I read a couple decades back. Retelling from memory (details probably wrong):
A boy waits in the schoolyard complaining about how long and boring all of this is when ??? appears to him and offers him a ball of yarn. Explains: This ball of yarn is your lifespan never let anyone see it. Simply pull the exposed string to make time pass by. But be warned, once pulled, it can never be returned.
At first he pulls on it lightly, skipping ahead to recess then to the end of school. Then he realizes, why not just give it a tug to skip the school year entirely until the next summer? As the story progresses, he keeps using it, finds himself suddenly drafted for war then back at home married. As an adult he realizes that time is all too fleeting and starts using it sparingly to help a baby get over an illness.
At the end, an old man, he laments how fast life as flown by when ??? appears before him again. He declares the ball was a curse and asks if he can have a do-over. ??? takes the ball and he goes to sleep one last time. In the morning he re-awakens back to when he first received the ball. Happy to experience life.