I don't think about this the way you do. Instead of looking back and thinking, "well, what I wish I had known..." I like that younger me, with all his ideals and optimism.
Instead, I look at my life now and think, "Would the 21 year old version of me be proud of the adult that I am today?"
Instead, I look at my life now and think, "Would the 21 year old version of me be proud of the adult that I am today?"