In the middle of a national diploma in public services and I have now realized I want to become a doctor.?

When I first started the course I wanted to be bacome a weapons systems officer in the RAF, then got told I could not because I suffer from megrains, I then decided to become a royal marine got to my medical and failed my eye test, and after paying �230 for a consultation at a laser eye clinic I was told they couldn't help me. So now I have found out that all the other public services (fire, police etc) won't allow me either so I wish to become a doctor or a surgeon and was wondering whether I should carry on with the course (18 months left) or go back to school and do my A levels. Please help (sounds soft) but I really want to do something with my life that will help others.