I hate, hate, hate, hate, hate the "stop and back" method of trying to teach a horse to slow down and "collect". This is super popular with backyard "western pleasure trainers" whose horses never hit the breed pen. It is not fun riding a horse that is expecting a stop every few strides and it does NOT teach a horse to properly round up and drive through.
Often you end up with a horse that is locked in the shoulders, neck, and jaw and a shuffling, short gait. Forward motion and drive is necessary to have a round, cadenced gait. You get drive by riding the horse forward into the bridle, not by making them back.
In addition, if you are on a horse that is easily intimidated or nervous, you'll end up with a horse that is expecting that back every single time you stop. Using a back as a correction is so wrong. Ever been on a horse that shoots backward anytime it thinks you are about to get after it for something? Not fun, at all.
There's a post in this thread I made about slowing down a horse's jog. You can use a martingale doing these exercises. Going in with your leg and driving the horse through and into the bridle is key, but especially important any time you add a training aid like a martingale. Need help slowing down horses jog.
Remember, that as a gaited horse, yours is probably going to struggle a lot with a jog and you may never get a good one.