How can i relief pain from my lower back?

I have been having back pain for the last 6years i have attended physio but there is no improvement so i stop the physio.Now the only way i manage to relief the pain is by placing abottle of warm water in the affected area which is the spine.Sometimes i'ts so painful that i can't bend.The doctors don't know what's the prpbles is.They say it's my height since am 6 fit tall.Pliz help?