I like riding in gag bits, or Pelhams the Pelham on a strong horse gets them to soften through their pull so you have a little more control I recommend a rubber straight bar for a horse that is soft in the mouth but a little strong at times, gags work differently they use pressure on the poll when you pull so your horse wants to come right back and get soft to your hand i also recommend a rubber snaffle gag on a horse who likes to be more foward through out the course, yet gags aren’t legal in the hunters or equitation so maybe only at home use only then put something softer in at shows if your not in the jumpers. Also if your at a show go early lunge your horse put them away so they are worked then they can mellow back out and be quiet for your time of showing.Hope this helped, good luck!
My horse used to do this- the only thing I found that helped was repetition, doing the same fence until it becomes boring. Serpentines into and out of fences are also good for slowing a horse down.
Also make sure you're not unintentionally squeezing your leg on the horses sides to grip or that your leg is not swinging back over fences, this was one of the main causes of our issues! A better jumping saddle did help.
I recommend working on half halts everyday. To do this, I like to do transitions to make my horse respect my hands and legs. My favorite transitions to make my horse respect my hand are canter to trot and counter canter to canter. These transitions have really helped my horse respect my hands and legs during flatwork and jumping. Hope this helps answer your question!
Hmmmm well it depends on how your horse feels when he is jumping. Is he green to jumping perhaps? He is galloping the course because he is scared, excited, or does not know any better or possibly a combination of all three. Canter poles and gymnastics help a lot for that too. If he is scared you will have to bring him back down to square one with just poles on the ground until you see and improvement in his confidence that the poles nor the jump itself are going to eat or attack him and he can conquer said evil demon. If he is galloping it because he is excited then make it boring for him. In between each jump toss a few circles and some figure eights here or there. As I said before toss some canter poles down. He could also probably reacting to your body language as well. Make sure you are sitting tall and thinking forward but relaxed. You should not need a ridiculous bit in his mouth to get him to listen. Make him trot into the first jump then do a circle or two and go over the next. I often do not practice lines with horses like this, I will take a jump then roll back into the next(after the circling/figure 8). Take it one jump at a time though. If he will not settle after the first jump keep taking He first jump until he does settle then go for the second.
I know this seems like a weird way of thinking about your problem but if you slow your mind down, your horse will slow down too. Another thing you can do is soften your eyes, when you squint you tense your entire body, which then causes your horse to sense your unease and rush. Finally you can set some nine foot poles in front of the jumps to back your horse off of them.
I hope this helps!