Finals, Lab rotations and stress

This semester I was told "oh if you're good enough at school to get into a MD/PhD program, med school won't be tough." I didn't work as hard as I should of tricking myself into thinking that I shouldn't have to work as hard as I did as an undergrad. Oops, my bad--finals week was stressful because I actually needed to get a decent score on the exams to get the grades I wanted...
But it's over, and now we're on to a new semester. Over the break, I went into a fellow mudphud's lab and worked with him a little, met the post-docs & techs, and talked with his Principle Investigator. I was thinking "this lab would fit me great!" But then I went and talked with another guy and met his tech and grad student and thought "this lab would be even better!" And then I had a little talk with the program director (amazing mentor in my opinion) who told me I shouldn't put aside my aptitudes (I graduated with a physics degree) since the most I'll have to flex my math muscles in the labs I'm looking at is what I learned in 6th grade.
So now I'm rethinking my summer lab plan, which is causing a different kind of stress and frustration than finals.
It feels like I'm trying to draw something I saw in a dream, but I forgot exactly what it was I saw in the dream. I mean, I have these ideas I get about solving a clinical question or aleviating a devastating disease with my career, but how I'll answer that question or find that cure, I don't know, nor do I know the disease.
Oh well... I have time--6 and a half years minimum, right?