Lower spinal pain for 11 years

When i was 10 years old(i am now 21),i slipped on a wet floor and fell straight on my back,i literally couldnt move on the floor for about 15 minutes,felt like i was paralised! Ever since that day i have had pain in my lower back,around the bottom of my spine. the pain comes and goes,i could go for weeks without having pain but yet even bending over,it would take me ages to get back up like im an old woman. when the pain comes though,it stays for days and i cant touch the area above my bum crack where my spine is because it is so tender and sore. If i am liying on my back in bed, and i want to move iv to do it very slowly because im so sore and stiff. iv been sent for xrays and scans by my doc but he cant find anythng wrong,just gives me painkillers but im only 21 and i feel like im 80 years of age.
If anyone could give me some insight it would be greatly appreciated.