Sorry, but I can never recommend more severe equipment for any horse to fix a problem. You need to work on your mare's training.
What this would entail, is going out on the trail with one or two good friends who are also willing to work on their horse's trail manners while riding in groups.
Next, you guys can plan and participate in excercises the will start to give your horses better manners on the trail. My friends and I work on stuff like this periodically, so I will tell you the kind of stuff we do.
1) Have one member of the group ride off at a faster pace while you wait behind. At first, they only trot off a few strides, and then wait you catch up at your own speed, then you can practice the same, ride away from your friend or friends a short distance and allow them to catch up. Gradually you can increase the speed and the distance with which people ride off. Also you can have just one horse move off at first, and then more horses move off, varying how many get left behind. Mix it up. This starts to train the horse that just because one or more members of the group take off, they are to listen to *you*. At first your mare will want to take off after them, horses do not like being left. But when it is a planned and controlled excercise, and you do it short distances and repeatedly, she will soon learn that she will not be left, and that she needs to listen to you.
2) Have your small group ride single file, at faster gaits, using constant transitions. What we do is, the person in front will call out the gait. So, say I am in the front, I call out, "Trotting!" if there are no objections, we will trot a few strides, then I call out, "Walking!" Then we walk a few strides, and so on. We add loping, stopping, just anything to keep the horses on their toes that even though we are going faster, it is controlled and they have to listen to us. If anyone's horse gets out of control, they call out "Walk!" and we will drop back to a walk to make sure that person can get it back together. Constantly vary your placement in the line, and who is the lead horse. Communication is the key with all these excercises, but especially this one. If you are in the lead, give everyone some time to prepare for each transition before you do it.
3) Play "catch up". Ride single file, at different speeds, starting at the walk and have the horse in the rear pass all the other horses at a faster gait to get to the front. You need a trail wide enough to do this safely, at least a two track trail. As I said, start off with everyone just walking amd the rear horse jogs by everyone to get to the front. Next you can try it with everyone trotting, then loping. It teaches them that just because another horse passes them does not make it a *race* and it helps rate their speed to either keep up with the group or go faster to get past them and then slow down again. You can practice this in an arena too.
If you are going to have a really nice, safe horse on the trail, it may take some work. You need to be riding with like-minded people who are interested indeveloping their control and safety, and ultimately better their relationship with their horses by doing these exercises. You have to be patient with one another. Nobody's horse will be perfect at these excercises at first, but soon you will have a mare that is really listening to you, rather than relying on pain to try to slow her down. Plus, these games are really fun to play.