"If I knew then what I know now, I wouldn't have wasted all that time."
This is a common thought for anyone who has persevered through some trial in life, and come out the other side with nothing material to show for it. Sometimes driven people can become so focused on their goals—particularly if it is a material goal—that they fail to address problems along the way.
Contrary to geometric reasoning, in human existence, the shortest distance between two points is not always a straight line. For one, navigating the minefield necessitates well-planned deviation from a straight path. For two, rough terrain may cause one to fall to one side or another even on a straight path.
Yes, a goal is vital and must be kept in view, but equally important is managing day-to-day concerns.
If you knew along the way what struggles you'd have, you'd likely never begin. If you did, you'd take a vastly different path which would bring its own unique struggles. It is from the struggles that we learn life's lessons.
If we knew all lessons, there would be no reason to do anything—if fact, there would be no way to do anything. Do you suppose anything worth having comes without some struggle created by an unlearned lesson? If so, what prevents you from having everything you've ever desired?
The truth is that when nothing material is gained from a struggle, that fact does not mean that the time spent in the struggle was a wasted. A series of lessons must contribute to developing the character within you that is worthy of attaining the goal.
If you knew then what you know now, then you know no more now than you ever did. Furthermore, you know nothing about learning from your mistakes—and are doomed to repeat them.
I pity the fool!