For me, I wish I hadn't second guessed myself so much. When I started working in the OR, I found myself enamored with the science of medicine. But as I continued working and started along the pre-med path, the responsibility of people's lives seemed huge. I knew I was smart enough to handle med school, but my younger, immature self wasn't sure he could handle the responsibility. And maybe I couldn't at the time, but instead of trusting that the process would prepare me, I hemmed and hawed for years.
Of course, I have no regrets. If I hadn't hemmed and hawed, I probably wouldn't have met my wife.