What Causes Acne On The Face, Shoulders, Chest And Back?

Hi I have been to a doctor about my acne. I get it on the face, shoulders, chest and back. They prescribed me some medication which i have been on for several months and they are not working. i went back to my doctor and they just said keep taking them. I've tried to change my diet to include more healthy things and less of the junk food but nothing seems to be working. People at school don't seem to notice, or if they do they dont say anything but it gets me down alot and really knocks my confidence. I have had it for ages and now that i'm 18 i really think i need to do something about it. What can I do?