Knee pain?

So im only 17, and I cant already be having knee problems. I was at a concert on april 26th, I jumped off a stage probably 4 feet high, and landed right on my feet. right as i hit, this huge shock of pain surged through my left knee and has hurt on and off ever since. Ive gone to a couple doctors, both assume its bruised cartilage, but would it really last this long if thats all it is? nothing helps it feel better (heat, cold, anti inflamitorys), except laying down. Any advice? its scary, because I plan on being a mechanic when i get older, but if i cant bend my knees, my future looks grim for that field.