I know how you feel; I was in your shoes, and have been for over 5 years (I'm 24 now). I finally decided to go take the 1 year Pre-Health science program (after hearing it was beneficial, and not only because it was a good "refresher" of high school courses, but it also gave me ADDITIONAL consideration into getting into the heath care program of my choice - the colleges like to see post secondary education and especially if it is a special prep program for the field!). It was not too hard (for me) but the hardest part would have to be the amount of material (notes and projects) we had to cram in our heads in such a small timeframe. It's ALOT of stuff to learn in one year... so if you apply and get in, try to dedicate as much time you can into studying and try not to work too much, or you might not do as well with all the pressure and whatnot.
I just graduated from pre-health and recently applied in London for the Western-Fanshawe BScN Nursing program. I hope to get in for this fall 2011.
It may seem like "a waste of time" to some people, but I believe that Pre-Health can be very rewarding, beneficial and give you an advantage when applying against secondary school applicants. It is especially great for people like "us" who are a tad bit older than average 18 year old applicant and need the better marks, but can't "re-do" a class or two.
Hopefully this can help ease your mind a little, and remember that you're not alone.. many people feel they cannot go anywhere because of their "bad marks", but there's ALWAYS a way out... or a way IN, I should say.
Also, if you have any general questions I'd be more than happy to help!