While I'm not a licensed psychologist (yet), I do have a Master's in I/O Psychology. I can't tell you much about the medical side of things, as that was only briefly covered during my undergrad, but I would first recommend that you start out by getting a degree in Psychology from a university that has a strong psychology program. Once you've done that, you will certainly be more well-informed about what choices there are wtihin the field, as well as if you will want to continue on to medical school.
I believe my psychiatrist was first qualified as a psychologist, and later went to med school (in fact, I think I started seeing him just before he finished getting his specialisation [psychiatry] finished).
EDIT: Oh, and if you end up not getting good enough marks to get into the course you're after straight off, remember that there are other ways, if you're willing to look for them and work for them. I know someone who actually started with a management degree and found their way into psychology

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Since you say you are smart but have difficulty with putting sufficient energy into your schoolwork, see if you can find a way to give yourself more of a challenge (extracurricular study, basically

)