I guess I realized a long time ago that living up to my potential would make me miserable.  I chose happiness instead.  I always tried to balance my life, and prevent any one element from consuming too much time and energy.  When I was studying engineering, I was also dating, exercising, creating relationships, playing music, reading for enjoyment, etc.  I didn't dedicate myself to my studies enough to make straight A's, and I definitely put a precedence on learning the material rather than maintaining a high GPA, so if an assignment didn't seem like it would increase my knowledge, I didn't spend a lot of time on it.
After getting a BS, I got a job.  I chose a job that wouldn't consume me.  I purposely picked a field I wasn't intensely interested in.  I put in my 40 hours, and go home.  My fulfillment lies elsewhere.  I met the most amazing woman and spent a lot of my time with her.  We got married, and started a family.
Now I have thrown balance out the window.  Now my kids are the focus of my life.  I make every decision with their best interests in mind.
A friend of mine from school went down a different path.  He got a phd, and started his own business.  I just celebrated my 5 year anniversary, and he's going to get married next year.  I have 2 kids, and he's been traveling the world giving lectures.  His fiancee isn't younger than him.  She doesn't have a lot of child bearing years left.  They'll be lucky to have 1 child.
When I compare my life to his, I just hope that he gets to experience the joy of fatherhood like I have.
I know I'm not going to make a huge impact on the world like this, but I don't care.  I don't think the world has any right to my potential.  The world gets as much as I'm willing to share.
Maybe after my kids leave home, and I have some money saved for retirement I'll think about making my mark.  Or maybe I'll just spend that time with friends and family.  I'll just have to see how I feel.