You're not staring death in the face but you're close enough to feel its influence. If not now when? That spurs you on, beyond any motivation you ever had at any age before. The experience helps, the realisation that those before you weren't any more special than you helps, but the ticking clock motivates like nothing else.
Here's my own personal motivator that has meant the most at this age.. — 'Most men lead lives of quiet desperation and go to the grave with the song still in them.' - Henry David Thoreau. That should scare the hell out of you, unless you're not old enough yet.