Time
Most of us waste far too much time. We fret about past mistakes, not wanting to make them over again. We worry about material concerns instead of what is really important which is enjoying the fleeting moments with those people whom we most enjoy the company of.
You can control more or less, how much money you have, how much stuff, how many friends. What you can't control is how long you have to enjoy these things. Everyday you go without seeing someone, loving someone, sharing time, is a day you can never get back. You can see them in the future, but the day you missed is lost forever...think about that...everyday you don't do something you enjoy doing you lose forever. You can never have that day again.
Starting right now, cherish every moment. Time will pass you by if you let it.