I have the same exact problem! My horse and I have been working on slowing down for some time now. What helps me is bending, it's EXTREMELY helpful. Good luck!
Transitions! Simple walk to halts (6 steps walk one halt etc) then walk to trots then same for canter to trot and then canter to walk. It's only a downwards transition, just you want it to achieve a walk and not a trot
takes a lot of muscle. work on lots of backing and softness and suppleness
I had the same experience with my young horse last and it was a very big problem to resolve for us ! To resolve this problem we worked a lot transitions: walk trot / trot walk until it was ok. After, we hadded the canter transitions. Also, we worked a lot lateral movements to improve the balance.
Try easier transitions like walk trot and trot canter. Extend his walk and trot for a few steps and then colect him back. After you have managed to get him calmed and relaxed during walk and trot transitions he should be more obedient when it comes to canter and his canter walk transition. Also when you do the transition make sure you sit up straight and deep in the saddle and when you ask for the stop do it more with your shoulders than pulling with your hands. Hope it helps.
Maybe try to collect the horse a little. Get his head down or practice troting for a few strides then go back to a walk and maybe keep doing that