Slow your posting and keep it steady at the pace you want to go, it helps if you count out loud 1,2,1,2,1,2 as it helps you to keep your rhythm steady, the horse will only trot to the speed you rise. Hope this helps
Maybe try to post really slow and if you post slowly he or she will slowly start to understand what u want them to do😊😊
Keep your heel off if he or she gets fast. And most importantly sit back and relax 👍🏻
Try to relax into the saddle and make sure you aren't accidentally tapping your horse with your heels. Maybe apply a little bit of pressure with the reins and when the horse slows down release the pressure to reward them and keep doing that until they keep a steady pace.