I think that this is a very good question.
Academically I was very well prepared. I had no trouble at all with any freshman or sophomore year classes.
I was not ready to work that hard without any letup for four years. In order to work that hard, you need to know why you are doing it. You need to want to do it.
By the time that I was in graduate school, I did understand why I wanted to do it, and I did want to do it.