We are in weekly lessons with my coach, and he's always been good in my lessons, she has me do sitting trot when he gets too fast and he responds well to my seat, our last lesson she gave us some homework to work on our canter trot transitions to do ten strides trot, ten strides canter and today after he got settled he did fairly well with transitions. Like I said, some days hes wonderful to ride, some others it takes alot of work
Personally I think you need to go back to a simpler bit without any twists. Teach your horse to mind and listen for your cues.. and respect them! Personally I would never put a twisted bit into my horse's mouth as that will cause him to react to pain rather than react to my cues of seat, leg, then hand.
If you get a harsher bit, it'll work for awhile.. then your horse will blow his aids... then he'll do whatever he wants, then you're back at square one. I think you both need a different approach starting with a softer bit. Get him soft again. There's no shame in lunging before you ride either. Get him focused and listening.
Is there a video you have of you riding your horse so we can see what's going on?
What I do to slow my horse down is lots and lots of transitions from walk to trot, trot to canter, canter to trot, back to walk.. or you can add some halts in there. Changes of direction, weaving, spiraling circles with a nicely bent horse (not overly bent or hind end swinging around), leg yields, any lateral work that you know, serpentines.
Add ground poles and trot over those. Look at reining and dressage patterns and do a few. Have a plan.. keep your horse's mind busy but sharp.
Personally I wouldn't sit trot, I'd post. Here's why.
When you post, all of your weight goes down your legs as there is no other place for it to go. It's a very clear "slow down" to the horse, especially when you manipulate the timing of the post.
Sitting trot has to be done when the horse is round.. when the horse is not round and strung out or blasting past your aides, you're more likely to start slamming on their back and causing more of an issue.. which creates a lot of "noise" and your slow down cues are muffled.
I hope you don't mind the advice