A horse that pulls, runs away, or goes faster than you'd like to go is not fun to ride, and a runaway horse can be dangerous. Controlling a strong horse may not be as simple as riding in a more severe bit; all horses are strong enough to pull a person out of the saddle.
Several factors may contribute to a horse pulling at the reins, including poor saddle fit, lack of outdoor time, dental problems, fear or lack of confidence, lack of training, a rider's heavy hands, and confusing or conflicting commands which lead to a horse's frustration. Figuring out why your horse is pulling away from you will help you determine the best way to fix the problem.
Once you have discovered why your horse is hard to control, you are on your way to a solution. You won't be able to force a horse to do what you want if it is feeling pain. Look after saddle fit, dental work, health issues, or hoof issues first. It's not fair or reasonable to expect a horse to be obedient if it is sore from any of these things.
Consider Digestive Issues
Keep your horse as "naturally" as possible. If it can't be turned out (allowed outside) all of the time on pasture to graze, be sure that it eats in a head-down position, and that it has ample time to be outdoors, and roll, run, and buck when it is feeling energetic. If its diet is high in concentrates, cut back while still providing good quality hay or pasture grass.
Check Rider Position
Sometimes we develop bad habits when riding and inadvertently teach our horses less than desirable behavior. These can range from poor seat position and heavy hands to mistimed cues and other habits that can confuse and frustrate a horse. Consider taking a few lessons to brush up on your riding skills.
Another thing to consider as you ride is if you are giving your horse a chance to anticipate what will come next. Do you ride quietly through forest trails and then kick your horse into a gallop when you come to an open field? Your horse may become excitable, anticipating what's ahead.
Retrain Your Horse
It won't be a mistake to take your horse back to the basics. Even if your goal is to trail ride, a strong, hard-to-control horse will benefit from schooling. The aim is to develop greater obedience. Ride circles, serpentines, loops, and other patterns at varying gaits. Keep your horse's mind busy and focused on what you are asking it to do.
You may want to start from the very beginning, lunging for obedience, and teaching voice aids. It's also essential to anticipate where a problem might occur and take steps to avoid it before it happens. If you're not sure how to go about this, a riding instructor or trainer can help you out.
Problems and Proofing the Behavior
Don't try to win a battle of strength with your horse; you'll always lose. If you feel your horse pulling the reins, keep a strong seat and leg and observe what's going on for clues as to why your horse is acting this way. Stay calm, talk to your horse in a low voice, and slowly bring him back to a walk if possible. Sometimes turning in circles will help re-focus the horse, or transitioning to walk/trot multiple times to get him more receptive to listening to you at a controlled speed.
If you don't keep a proper grip on the reins, they become too long and the horse will be able to pull away more easily. Remember that your horse's head moves slightly with each step, so you should keep your arms at your sides, and allow them to move with the natural motion of the horse's gait instead of letting the reins slip through your fingers.
You need to find the balance between giving your horse enough rein and gripping too tightly so it becomes confused.
A horse that chronically takes control and pulls too much forward may either need a stronger, more advanced rider, or may need serious professional retraining. If you have recurrent issues with your horse, talk to a trainer for more one-on-one guidance.
Equine Nutrition: Concentrates. Utah State University Cooperative Extension.