Sounds like you've got yourself a pretty forward Mustang and I have trained a few Mustangs myself that fall into this category. There are basically two types of horses: one with too much go and one with too much whoa. I just finished a Q&A to someone that has a great Western Pleasure horse, but now she wants to ride English and is frustrated over trying to make the horse move forward. Your horse has the opposite problem and although both are frustrating (for both you and the horse) in my opinion the forward horse is a little more challenging to deal with.
We get a lot of forward horses in training, because they are more challenging, so I'll share with you some of the techniques that we use to get the horses slowed down a little and to get them steady in their gaits. You'll never make a Western Pleasure horse out of him, but you should be able to get him to hold a steady pace and slow down and relax.
First let me tell you what definitely will NOT work. It will not work to use a harsher bit or to constantly pull back on the reins to slow the horse down. Any pressure on a horse's mouth makes him more anxious. Forward horses tend to be more anxious to begin with and they also tend to be very sensitive. More pressure on their mouths almost always makes them faster. The classic scenario with a forward horse is that the rider is constantly pulling on both reins and the horse is getting more and more tense and starts to jig and speed up.
What we want to do with a forward horse is anything that will make him relax, put his head down and slow down. We want him to learn to make an association between relaxing and getting a loose rein, which is what all horses want. Since we cannot slow him down by pulling back on the reins (because the increase of pressure just makes him more anxious), the best option is to work on changing directions. Often people talk about using the circle to slow a horse down, but I find reverses to be much more effective.
For one thing, often when people use a circle to slow a horse down, they pull the horse into a sharp sudden fast circle, which tends to excite and irritate the horse, throwing him onto his forehand and unbalancing him, rather than slow him down. Every time the horse changes direction, he has to slow down as he turns back the other way.
For changes of direction, ride two-handed in a Myler Comfort snaffle or the Myler 3-ring Combination bit, with both hands well in front of the pommel and with a reasonably loose rein. In slow motion, move both hands to the side (in the direction you want to turn) so that the inside rein is a leading rein and the outside rein is a neck rein (see the Q&A about rein-aids). Move your hands as a unit like they are connected; there is NO BACKWARD PULL ON THE REINS.
Do not pull back on the reins and do not try to slow the horse down. Just turn to the left for a moment, and then slowly and gently change your turn to the right, then left, then right, etc. It is critical that you are moving your hands in slow motion and that you are not pulling back on the reins as you do. Your turns should be totally random, half turn, quarter turn, full turn, etc.; don't let yourself fall into a pattern. You should flex your horses's neck to about a 45 degree angle as you turn.
Gradually (it may take minutes, hours or days) your horse will begin to slow his trot every time he changes direction. Once this starts happening, let him go straight between turns and then slowly and gently bring him into a turn when you feel him first begin to speed up. Ultimately, you should be going straight between every turn, and then you will let the horse stay straight as long as he is relaxed and slow until he is maintaining a steady relaxed trot without turning.
Make sure your hands are moving together and there is very little or no pressure on the horse's mouth. Whenever you need to correct the horse's nose to bring it into the turn, pick up (not back or down) slowly on the inside rein, with a gentle bumping motion and immediately release it when the horse gives his nose. Eventually, you should be able to make these turns and straightness with just the slow movement of your hands on a totally loose rein and without actual contact on the horse's mouth.
Another thing it would be very useful for your horse to learn is a drop-your-head cue. With nervous horses, this is a very important step. A horse's head comes up as he tenses and it drops as he relaxes. If you can cue your horse to drop his head, you have succeeded in teaching him a cue to relax, since it is not physically possible for him to drop his head and stay tense.
Start from the ground with a rope halter and training lead. Apply gentle steady pressure down on the halter by pulling on the lead and watch the horse VERY carefully for his head dropping. At the very first fraction of an inch, release the halter and praise the horse. Then ask again, watching very closely for any movement in the right direction, then release and apply copious praise. It is better to err on the side of the release being sooner rather than later. Timing is everything in horse training. The optimal timing for a release is half a second after the desired response. The first few inches down will be a challenge, but the next foot is easy. Once the horse figures it out he will gladly drop his head all the way to the ground every time you ask.
Once he can do this reliably from the ground, it is time to teach the same cue from the saddle. Standing still, you will use one rein, shorten it up to apply light pressure to the horse's mouth and wait until the head drops the smallest fraction of an inch then drop the rein entirely and give copious praise. Don't worry if the horse begins to move about, just focus on the head dropping and release whenever it does. In the same process the horse will gradually figure out that when you apply pressure with one rein and he drops his head, the pressure will go away. Again, the first few inches are very hard to get and will require a great deal of patience and concentration on your part, but the next couple feet to the ground will come much more quickly.
What ever you do, don't succumb to applying more pressure if the horse does not respond. Apply just enough pressure that the horse begins to look for a way out of the pressure. He will guess what to do to find the release. At first, he may put his head up, to the side, etc., but eventually he will try moving his head down and that is the instant he must find the release so that he makes an association between the cue and the right answer. This is a general concept in training that applies to almost anything that you do. More pressure is not usually the answer; you just need enough pressure that the horse begins to look for a way out of it.
Oddly enough, sometimes a very nervous horse can actually become addicted to lowering his heads and relaxing. He doesn't really want to be nervous and frightened all the time and when he finds the peace by putting his head down, he comes to like it. This can actually become a little bit of a problem in some horses (they want to put their head way down all the time) but that is not very common and it is a far better problem to have than the original one.
I would not expect your horse to become a Western Pleasure horse, but he is capable of slowing down, relaxing and getting steady. Try these exercises, be patient, always move your hands in slow motion and give the exercises considerable time to work. Good luck and let me know how it goes.
If you liked this article, Julie suggests the following products to help you continue the work with your horse (or call 800-225-8827 for ordering help):
Goodnight's Principles of Riding, vol. 1 Balance & Rhythm: http://shop.juliegoodnight.com/shop/trgprbalanceandrhythm.html
Goodnight's Principles of Riding, vol. 2
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Goodnight’s Complete Rider Package: http://www.shop.juliegoodnight.com/shop/packagecompleterider.html
You may also enjoy working with Julie to help conquer your goal at a Clinic Tour stop. Click here to ride or watch a clinic. http://juliegoodnight.com/clinics