With my trainer she had me trot for what seemed like an hour straight until I got my gelding to trot slow. Now I'm talking about an Appy who has stamina through the roof, so I couldn't have ridden him until he was tired if I tried - and that wasn't the goal, either. One method to get them to slow down is working on walk/trot transitions. Seeing as your horse is gaited I'm not entirely sure how that would work. But my trainer would have me trot a few yards then cue to walk a few yards then pick up a trot again. It worked after a few time because he was expecting to walk.
Another thing she had me try was letting him trot on a loose rein, then slowly brushing up each rein alternating and putting light pressure on the rein. With that you must relax and sit back, moving and thinking 'slow'. Don't be squeezing with your legs just breathe. Again, this had worked but my butt doesn't approve of this one. (It took a while the first time, but after each time he catches on a bit quicker.)
Another is to use obstacles. Take the horse on the wall and ask for a trot. Lean back and just relax, and see your horses response. If she continues to walk fast you simply take her into the center of the arena and do circles or weave cones, barrels, go over poles. If that slows her down, then take her back out onto the wall. If you get a few strides of nice slow trot then praise and ask for a walk. Also you can do a large figure eight, and ask her to walk in the middle, then pick back up when you get to the edge. This is the same as the walk/trot transition where they anticipate the walk and might give you a baby trot.
I am not say these absolutely will work, but I've tried them and they did work for me. It really depends on the horse, as my gelding will trot for 30 minutes straight doing these things, then all of a sudden it will click. But I love posting, so it was really hard for me to sit and convince him that I was seriously going to go slow. Just see what works for you and be persistent. Good luck. ;)