If you are learning how to stop a horse you may have been taught to pull back on both reins.
Yikes! This is one of the biggest errors still being taught to riding students worldwide. I learned it, too.
Maybe your horse doesn't want to slow down and you are using a stronger bit. Or perhaps he tosses his head and you've put him in a martingale or tie-down to keep his head down.
These are only symptoms of a much bigger issue.
In the picture above you'll see the typical way riders are taught how to stop. Pull straight back on the reins. This only works to a degree. He can either grab the bit with his teeth or completely use his powerful hindquarters against your wish to slow down.
A more effective way to stop a horse is to teach them first to follow the energy level in your body, then use your seat cues to slow down, and then one rein if he doesn't stop.
It is much harder for him to run away with his neck bent. Bending the neck also moves his hindquarters away from the side that you are using the rein on, which makes him less powerful.
Here you can see that my partner willingly bends her neck at my request. This is true in any situation and is a great "emergency" stop technique.
Here you can see the placement of my hand going towards my upper leg.
If you are having issues stopping your horse you don't need stronger bits, you need more training and preparation and it starts with the above and a solid groundwork program.