What I learnt from David O'Connor is "Your horse will match your seat".
So, if you slow your seat down, your horse will come down as well to match it.
And what I learnt from Dorothy Crowell - is "Seat Into Legs Into Hands To Soften"
Exactly as CJ8Sky said:
regulate pace and rhythm with the seat, not hands.
I know it'll be very difficult to not use your hands to slow your horse down, but really try hard to think about using your seat first, and what your seat is doing.
Slow your seat down, and your horse will follow suit :)