Hello Jessica and welcome to the forum,
Try to use a twisted D-ring snaffle, they seem to work the best from my experience, but a lot of riders like different things that work best for their horse, you'll find out what works easier, and it sounds like you have to go back to horsemanship, work with your horse at the walk first, work on your stop's roll backs, flexing, and counter arcing, when you trot up to your barrels and go through them slow, trot up, when when you give the cue to slow down, really sit in your seat like you do and walk the horse around the barrel, when its time to speed up again, give your horse the loose rein again and really ask for that trot and when its time to sit down into your seat, sink in and ask for that walk again. for the loap, do the same thing and trot around the barrel, you want to practice perfect. A video would help a lot if you can get one, but just really sounds like you have to really go over that slowdown cue again, and you really don't want to say "whoa" unless you want your horse to stop, for a slow down cue opposed to a stop cue you should use the word "easy" or the horses name in that kind of "easy boy" or easy girl" tone. Good luck and let me know how you do:wink:. You should never have to be hard in the horses mouth, oh and on those barrels really apply a little pressure to your inside leg, it helps your horse bend around the barrel, and never look at the barrel, look where you want to go, because horses can sense where your looking. Again, in between the barrels you don't want to be in the horses mouth at all, unless you really need to, that way when you really need to use the reins, its going to mean something. Right now I'm using a regular D ring snaffle, and don't do the pattern every day, your horse will get bored, always change it up with trail rides or doing a different activity, that way your horse is always excited to do something new, and he'll always want to work with you!! anyway good luck and keep us posted on how you do!