Bear with me on this:
You're a point (Y). Off in the distance somewhere is a line that represents your death (D). Two other lines (E1, E2) -- the extremes of what you can do with your life -- radiate at an angle (A) from Y, eventually crossing D.
Now, time -- cruel, marching time -- is constantly pushing Y forward, shrinking the distance between where E1 and E2 cross D (assuming A remains constant). If you spend ten years walking the straight and narrow of E1, then the place where E2 originally intersected D is gone, forever and always out of reach. You could spend the rest of your life heading along E2, but you'd never quite reach where you might have been.
I spent Saturday afternoon sitting on the lawn in front of my house with my one-year-old son. It was a glorious Los Angeles day, clear, slightly breezy, warm. The neighbors' tree shaded us, and Tom bounced around the grass, yanking it up and tossing it away. He pulled himself up by my shirt and grabbed at my glasses and smiled and laughed.
Sometimes I wonder about E2, about where I might be if I had made different decisions. Happier? More adventurous? Kinder? A better person? If the whole of possibility was spread out before me again, would I still end up where I am now?
Tom blinked in the sun as an airplane passed overhead, and then reached out to grab at it.
And everything else I might have been was forgotten.