You don't ride with your hands, you ride through your seat.
You slow your horse through your seat. You obtain control and rhythm through your seat.
What will really fix the problem? Dressage, Dressage, Dressage, Dressage.
I had the same problem with my horse, she also just used to go straight through my hands. My riding instructor says that she was deliberately ignoring my hand aids and not respecting them.
It was because your horse was unbalanced and your horse was trying to obtain that balance through their neck, not through their core and their back end. But that also means that your horse was not getting any help from you, the rider through your position, seat, legs, upper body.
You do not slow your horse down through your hand aids. You do not slow your horse down by pulling on their mouth. You do not slow your horse or obtain rhythm through your hands at all.
That is ass backwards riding.
This may of worked for your horse, but my question is - is your horse truely balanced? Engaged? Back end dropped, ribs lifted, off their forehand and light up front?
This is something every rider out there should know
SEAT into LEGS into HANDS to SOFTEN. It is taught by Olympic Eventers. It is taught by Olympic GP Jumpers and it is definately taught by Dressage World.
To have a horse balanced, rhythmic, fluid, round, soft, supple comes from proper Dressage Work. Proper balance from their rider.
If you are NOT Balanced. NOR is your horse.