I had rotten school habits when I hit college and, in combination with the new-found freedom, my grades sucked. In almost any field other than medicine, great grades in the last couple years will overcome a bad first year or two. So I had no trouble getting into grad school. But you need to hit the ground running if you're aiming for medical school, which means you have to challenge yourself now. That way you'll have good habits before the grades start to really count. And don't be thinking you'll be a failure if you don't attach MD, PhD, or other letters behind your name. You can't predict what you'll be wanting to be doing in 5 years, let alone 20, so don't panic.