Think back to something you looked forward to. The anticipated day, event or moment that seemed like it would never arrive. But it did, and all too soon, it passed. Some of the best days of your life are now a pleasant memory.
Even some of the not-so-pleasant memories you have have lost of their sting. That friendship that ended someday brings a rueful smile instead of a grimace. That hard time you thought you'd never get through eventually ended and you came out stronger for it.
But sometimes it's hard living in a world where nothing stays the same. Nothing is in your control, you have the power to do nothing that will last. Anything you accomplish will one day be destroyed, you can't make good times last forever, and try as you might, you can't make hard times nonexistent or even go away just a little more quickly.