What could be the possible cause of my knee pain from an old injury?

A little over a year ago I got injured, drove my knee into a wooden picnic table at a fast rate of speed. It swelled, bruised, the works. I made my way to the doctors eventually to get it checked after it not having improved. I was told I bruised it, they sent me home on crutches, and a big blue brace. At the time it hurt to walk, to bend my knee and to have constant weight on it. Over a year later, at this point in time I am still having problems. I have pain in certain spots of my knee to the touch, never is the same, it goes from one spot to the next. I also have slight pain when standing too much, driving or walking too much, as well as when I'm doing nothing more than sitting. I still to this day cannot sit on my leg with it bent, and sometimes when I do its almost if it locks, and I quickly have to get up, or as if it is going to pop out of place. Its very painful at times. Its not something I can ignore, but after going to the doctors, I'm not sure if another trip there will help if they will only tell me the same. What could my problem be? Should a bruise still be bothering me over a year later?Thanks for any help, its greatly appreciated.