Has anyone taught you the one rein stop, or the concept of disengaging the hind end? After many years of leasing, I bought my first horse, an OTTB, at the age of 40. I was more than aware that getting hurt was not an option since I am my sole income provider. My horse is a saint for the most part, but he has his "happy" moments, his share of spooks and times when he really wants to catch up with the horse in front of us on the trails. I found out something very powerful that horses don't seem to know. That is, unless we prove it to our horses that we cannot control them, they don't know it. How do you control them? It's all in the hind end. If you can, have your trainer show you the one rein stop. All it is involves taking off all leg pressure, sitting deep, letting one rein go loose while pulling the horse's head around with the other. Practice it at the walk, then the trot, and when you get really good at it, the canter. The idea of the one rein stop is, if you take away the horse's ability to run off on a straight line, you take away their power. Most horses learn the concept very quickly and will come to a halt as soon as they feel you start to take up contact on the rein. Once they know you have this move in your tool box, they quickly loose interest in bolting. You have to catch them before they get into full bolt mode or else you can throw them too far off balance, but it's safe to use within the first two steps or so. At worst, you make a big circle instead of sudden stop. Just use your head. Sorta fast = turn him more. Way fast = big circle.
Another tool that is very usefull is a dressage move called shoulder fore. I highly recommend learning it. If you're just walking your horse around the ring or on a trail and even think he might bolt, shoulder fore puts one of his hind legs directly under his body and again blocks straight line movement. It's a very valueable tool.
I can tell you for certain that both are extreme confidence boosters. The proof is in my ride last week. I was out on a trail just walking along like all was right with the world. Out of nowhere, my horse did this major league spook. He spun around and tried to take off. Within the few very strong strides, I dropped the one rein and just started to pull on the other and he stopped on a dime. He was still very tense and jiggy, but I was absolutely amazed at how all the training we had worked on created such an instant response. I then had to use shoulder fore for the next 100 yards or so to get him past the monster without bolting again. He wasn't thrilled about it, but I had full control all the way home. HUGE confidence booster.