A clock.
At first, when I read the question, I thought it was going to be difficult to answer but then I heard the ticking of a nearby wallclock in the background and then the story/answer revealed itself automatically inside my head.
Every second, we grow older. We don't usually put much thought into this but with every tick of the clock, something within us changes. And just like every clock, as time goes by, its battery runs out, and dies. So the ticking stops.
My clock, the one which symbolizes my life, has had its batteries changed a lot of times. And when I feel that there is no tick-tocking, I feel dead inside. The good news is that there are a lot of batteries to replace the old ones and make the clock work again. I always need to remind myself, everytime my battery dies, to change it for a new one. If I ever took my time in changing them, there may be problems to arise in my clock. The hands could get rusty, and the interior of my precious clock could be broken.
But as sure as I'm taking my next breath, I will be sure to collect as many new batteries as I can. I'd advise you to do the same thing, too.