I am not an expert but I do ride gaited horses and have trained a few young horses. One of the things I have noticed about a horse that is pacy is that they have not developed appropriate muscles to help them to gait correctly.
Here are some things that I have done to help my horses to develop his/her gait. I start out by getting my horse in shape by getting him limber, flexible and in condition. Ride or lunge your horse every day to condition him/her. When riding, work on bending exercises such as small circles and weaving obstacle courses to make the horse more agile and flexable. I also go up and down a lot of hills at a walk, that will also build muscles and get them in shape. I also allow my horses to canter. Yes, cantering your horse will help a gaited horse develop his gait. It took me a long time to figure that out but it is true.
I start by asking my horses to walk, my goal is to "walk" my horse into the gait. I walk my horse as quickly and actively as I can without letting it move up into a trot. I like to see a good head up and down forward movement. I use my legs and pelvis to maintain the forward movement. I rock my pelvis with the horse's movements, encouraging him to walk more actively. I practice maintaining an active walk. You may wish to use an impulsion aid such as light spurs or a crop. Don't hit your horse; just tap it lightly on the shoulder or flanks to encourage a more active walk. Many horses like to speed up for just a few steps, the return to a slower pace. I make sure mine does not have a chance to go int a slow pace. This is what will help him develop the smooth gait your are wanting and is well worth the effort.
Yes, you are to get the horse collected. This means that its center of gravity should be shifted backwards. One way I help teach my horses to collect up is to halt, with me shifting my weight backwards in the saddle and applying leg aids at the same time. Just like someone else has said you should put some pressure on the reins, keeping your hands low. This should help shift the horse's weight onto its rear legs.
Continue asking for more collection and speed. Use the half-halt to request greater impulsion and collection. You can execute a half-halt by asking the horse to halt, then propelling it forward the minute it begins to respond to cues, Soon, your horse should move naturally into the gait it was bred for.
Make sure your horse is performing the right gait. Look at your horse's head and neck. It should continue moving in the steady up-and-down of the walk, not the back-and forth movement that means he's pacing.
Now, be patient keep in mind it takes awhile to build their muscles and get them in the appropriate condition so that they can perform the smooth gait that we desire. Just enjoy your horse.
I hope this helps.
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