How to fix this galloping issue
My horse is a 15yr TB, has been off the track 11 years, and has been dominantly a ring horse with some controlled walk/trot/canter on trails and a schooling field. I haven't had the place to take him out and let him gallop until very recently, when I moved him to a boarding farm. He was intended for hunter/jumper, I saw he had tons of holes in his flatwork, so I did mostly flat reschooling for a while. He now goes beautifully in a fat double jointed copper loose ring in the ring to flat and to jump, and also goes in this bit to hack out walk/trot/canter by himself.
Recently I've been taking him out for hacks around the farm, the majority of the path is around the edge of a big hay field (very open). He is not barn/buddy sour in the least, so that's not the issue. He's very good to trot out for the first bit, has a good, controlled, balanced, and fairly slow canter when I ask, and up until yesterday had a pretty good gallop. I'd let him go, encourage him on, and he would stretch out low (he isn't a high-headed type horse). The slow-down would be a bit tricky. I could get him into a definite slower balanced canter, but it would take a while before I could get him to drop back down into a trot or walk (once he breaks down from the canter, I can put him on the buckle and he'll walk or halt- no jigging). I could feel him start to brace against my and and sitting down in the saddle and half-halting did nothing. Pulling obviously wasn't working, so I'd have to plant a rein and yank repeatedly on the other (that sounds really ugly but it wasn't THAT terribly bad). After the first couple times of trying this in the snaffle, I switched him to a double jointed two ring elevator (using two reins). The extra rein on the leverage part hardly made a difference- he still tries to lean. I was okay with this because I figured I could gradually school him to listen a bit better.
We can still do short canters after this, in a different spots, and he's fine. We canter or trot in a lot of spots where we can get short ones in, but only gallop in this one stretch because it's the only area long enough where I can really let him open up. I know that's part of the issue, but I KNOW he knows we're going to gallop there. He will walk fine, but when I trot coming to the stretch, I can feel him "asking" to go. I'll cue him, he'll go into an open canter, I'll give him a kiss and some leg, and he'll gallop. Usually we only do it once per ride, coming from the same direction, and then he's fine to do a short canter after that in another spot. So I know it's this one stretch. The day before yesterday, I turned him around after our first gallop, and let him gallop back along it. When it came to slow down, he wouldn't (as usual). I could definitely feel the bit working to lighten up the front up so he wasn't pulling down against me, but I had to put him sideways and use a lot of effort before I could get him to slow down. And this was coming into the woods- I doubt I could have gotten him to slow down in the open. Even still, it didn't seem that bad.
Until yesterday. We were doing our normal gallop in the only spot where he can really open up, and it turned into a very open, very fast gallop with him REALLY stretched out. I tried to slow him down through my seat- nothing. Tried to slow him down through my reins and he pulled down and went faster. That's when it got scary. I tried a one rein stop, tried both reins, pulled on the snaffle and the curb reins as hard as I could, he just stretched down further and went faster. This, I imagine, is what his race training taught him? I had to really grab one rein, sit down, and pull as hard as I could into a circle in the middle of the hay field until he had to stop. Not easy and not fun.
I didn't at all expect that. It wasn't technically like he was bolting, because I asked for a gallop, but I definitely did not ask for that open and fast of a gallop with zero input from me. I didn't gallop, canter, or even trot him after that. We just did a long walk to cool out (which he was perfectly fine for on the buckle). Usually our first gallop is fine, just the normal issue with completely coming down from a canter.
Now I'm thinking he's probably going to do it again next time, which I want to prevent, but I still want to be able to let him gallop to stretch out. I know many people will say reschooling, but I can't school him if he's running away with me and I can't stop him at all. The mild mouth-piece two ring elevator (second rein is on the first extra ring) obviously doesn't work. A martingale would do nothing because it's not a high-head that's the issue, it's him stretching down and just running. When I was starting to show him in baby hunters several years ago (without good flatwork- he would try to pull down against you when jumping), we put a bit with a corkscrew mouthpiece on him because he would get strong, and even then I could get a slow slow slow canter out of him, but could not break down to the trot. I don't know what to use on him and would love suggestions on how to fix this problem. I know the horse very well and he's a bit tricky in general to figure out his buttons- which is why in general I've had more success in having a cooperative, smooth ride than some of my trainers (trainers have been on him to teach him new things but he's never as relaxed or cooperative with someone else up as he is with me, even doing basics). Point I'm trying to make is that I'm not overhorsed, we make a good pair, but I want to try to event him and can't do so unless I feel I can have a safe, controlled gallop with brakes. I need some new tools to try on him and hopefully you guys can help!
Sorry for the super lengthy post, I just wanted to include all the details.
ETA: He hates anything with a curb chain and goes in a figure-8 bridle.