I also suggest having your horse checked out for any sources of pain. I knew of a horse who had always been a calm level-headed one even for children to ride. After a seemingly minor accident the horse afterward would run uncontrolled as soon as she was mounted. Radiographs were eventually done and it was found that the mare had fractured a front cannon bone and had to be put down. I hope for you that your horse is sound. Odds are likely that he is sound just needs some back to basic training. Letting a horse run thinking that it will tire-out the horse is just putting cardio-vascular and lung capacity conditioning in the horse. The suggestion of working and riding in a small enclosed area to begin with is a very good thing to do. It sounds like you and your horse would benefit from learning the one-rein stop if your using a true no leverage snaffle bit.
As best as I can I'll try to describe the one-rein stop. As soon as the horse takes off at the canter/gallop take up the slack of your right rein and lay than hand/rein on the horse's mane/neck then with the left rein/hand pull your rein/hand to your chest. What will happen is that the horse has no option but to slow down and come to a halt. If your bit is a curb bit with jointed mouth you can pull her head to your knee and your horse will have no option but to circle and come to a stop.