'Never Get me back' riddle

I can run swiftly and silently when you want me to stay still,
I can move slowly and cautiously and am yours to fill.
You look at me often and yet you always forget me,
I am the most feared killer, yet you can't live without me.
Sometimes you have me for all to spare,
Yet when you need me, I am not there.
You can waste me, or cherish me, you choose the track,
But once you're done you can never get me back.

What am I? Any guesses?