There is nothing more uncomfortable than having your horse stop on his front end! If your horse is stopping on his front end (or not stopping at all), he's not engaging the rear-end.
Teaching your horse to collect and engage his rear will provide a better, more comfortable stop.
A simple way to teach your horse to stop better Start Show: From a walk, ask your horse to stop using your reins and the voice command "whoa".
Immediately upon stopping, ask your horse to back up several steps.
Ask for a moderately aggressive back and use your legs (remember, your legs are the gas pedal going forward or backward - don't maintain constant pressure just "bump and release").
Additionally, don't use steady rein pressure to ask for a more aggressive back, use two hands to alternate between the left and right.
Repeat this several times, each time your horse stops, immediately ask them to back...
each time increasing the speed slightly in reverse.
When you ask your horse to back, it is more natural for him to collect and engage the rear, therefore, by backing up after a stop, your horse will learn quickly to get ready to back and engage is rear-end.
Desired Result: You should feel your horse start to engage his rear and even take a step backward without being asked.
Additionally, before moving on to the next step, your horse should stop with the voice command, "whoa"! Next: Repeat the above steps from a trot.
Even though your horse is stopping on the voice command "whoa" at a walk, reinforce the techniques above for the stop and back.
After you have your horse stopping on voice command and engaging his rear-end at a trot, move on to a lope! Desired Result: You should be pleasantly surprised how fast your horse will learn this! Remember to reinforce during regular training sessions as needed.