Having Knee Problems...?

Ok, so over two years ago I had gone to the doctor to find out what happened to my knee following a car accident, a hockey incident, and momentary blowing out of my knee. With all this happening within a month, there was considerable pain and I have since lost a lot of usefulness with it. Now, the doctors did an MRI and X-Rays, but no damage was found, and they suggested with time and conditioning it should be fine. However, it has been, as I said, over 2 years now, and the pain is still there, and though I have learned to get around rather normal day to day, it is still painfully obvious that whatever happened hasn't healed. Can anyone give me a suggestion as to what is wrong with it if no damage was found after MRI and X-rays were conducted? The pain tends to stem in the back of my knee, my left knee, and occasionally is felt on the inside and the outside of the joint. Thanks for any help.