A Month of Summer by Lisa Wingate
"Life is a journey by train. Outside the window, the scenery is rushing by. If you look away even for an instance, something passes uncaptured. Far in the future, when you leaf through the photo album of memory, your finger, aged and crooked, will rub lightly over that empty space and you'll wonder, what might have been there. In your daydreams, you'll return again and again, try to open your eyes, for that single moment, but you can't. This is the science of regret according to Claude. Life is a journey by train, and the engine's always at speed. Don't close your eyes, even for a moment."