Opportunity cost. For every choice you make you are sacrificing something else. The thing you didn't choose, the road you didn't take.
It's all I can do not to dwell on the choices I didn't take, for better or for worse. There's no-one to help me choose, no-one to show me the way if I get lost down the wrong path. My life is defined by the sacrifices I make, by the roads I take.