Okay, well I know no one likes the cliche advice I'm about to give, but please use it: "Take it one day at a time". Priorities change over time, and the late teens and early twenties are a time where you will fluctuate constantly about what you do and don't want to do.
Take something that will leave options open (including a back up plan). I'd say take general biology, general chemistry, and either calculus or physics your first year. Take as many arts electives as you can (they are a major stress reliever).
I was a high school "hot shot" like you, I got to University, and I hated it. I went to school for two months, withdrew from everything, and took the rest of the year off before reapplying. I reapplied, got back into school, took biology/chemistry/physics/calculus/english both semesters and I've got a 4.18/4.00 gpa now.
Don't put unneeded pressure on yourself. Go through your first year, and see what happens. If you have the marks start planning for actually getting into a M.D., but the first year is really a litmus test to see if you like the sciences enough to compete and do well in them.
Good luck, be a 18/19 year old for a while, and don't stress out too much.