Bad answer:

I never finished law school and everything that has happened since then has taught me that giving up, just because the going gets tough, is a huge mistake.

You do not want to actually highlight a major regret especially one that exposes an overall dissatisfaction with your life. Instead, focus on a smaller but significant, mishap and how it has made you a better professional.

Good answer:

When I was in college, I took an art class to supplement my curriculum. I did not take it very seriously and assumed that, compared to my engineering classes, it would be a walk in the park. My failing grades at midterm showed me otherwise. I would even jeopardized my scholarship status. I knew I had to get my act together. I spent the rest of the semester making up for it, ended up getting a decent grade in the class. I learned that no matter what I am doing, I should strive to do it to the best of my ability. Otherwise, it is not worth doing at all.