My Pony Keeps bolting?

Back in September 2006 I bought a section C Welsh Cob he is 20 years old and came from a lovely home. He is a real sweet heart and is a genuine kind hearted pony. When I first rode him he was very respondent and was a real dream to ride. At that time I was still a novice rider and he seemed to be very sweet and slowed when asked. I started to have riding lessons on him and one day decided to take him out of the paddock into the big field (big mistake). He was trotting fine and I then asked him to canter, he just took off with me like a steam train I couldn't slow him down. I ended up falling off him as he turned so sharply and I ruptured my knee and fractured my ankle. Since then I am very nervous of him and have had several lessons on other horses to try and get my confidence back and make sure my riding skills were up to scratch as I felt it was my fault that I bolted. I am now an intermediate rider and I have only been riding him on the lunge rope, last weekend I decided I would take him around the paddock he was perfect he trotted when asked and cantered and slowed when asked. I took him up and down the field several times and then again he took off into a fast canter, then galloped out of control I was thrown at full speed to the ground and I now have cracked ribs. I am teriffied now to ride him incase this happens again. I try and ride him once a week, during the week he is kept in the paddock and fed on a mixture of nuts and hayledge. He doesn't appear to have a sore back and when I put the saddle on it doesnt seem to bother him? I dont know what else to do please help.