I agree with meagan_davis. That is exactly what I would do, every time she starts to tense up, BEFORE, she bucks, take her and circle her, make her work her butt off. Then she’ll start to correlate “okay, if I buck, I have to work really hard, and I don’t wanna do that, so maybe I just won’t buck…” but again, make sure you catch it before she actually bucks. Start off with a few circles, then increase the amount you do every time, because, as always, start of with a little pressure and gradually increase the pressure, so they learn to respond to as little of pressure as possible. Doing some round pen work with her would also help you teach her some respect. I can’t go into too much detail about it, because that would take forever, but there’s a lot of different lessons to be learned from round pen training/basic respect training. But, and don’t take this as me insulting you or anything, if you decide to do round pen training, I would get someone who knows how to do it, either do it for you or help you, because there’s a lot more to it then following a technique, if that makes sense.
As far as the reactive side of her brain, it’s something that are born into all horses, and is there when they’re young or old, it doesn’t matter. Horses are a flight animal, not a fight. This also means they’re a reactive animal, rather than a thinking animal. So their first instinct is to react. Our job as riders and trainers is to teach them to use their thinking side, make them look for answers. We can do this through pressure and release. By applying constant pressure, and holding that pressure as they work through their options. For example, take a horse that wants to go one way when you want to go the other. When you put some sort of pressure on them as they go the way they want to, they’ll automatically want to look for a release of pressure. So, they try going all the possible directions they can go, except for the way you want to go, and you keep holding pressure in whatever form you’re giving pressure, and they start to think “hmm, over here doesn’t work, and over there doesn’t work, what about over here where she asked me to go in the first place….” and when they go that way, you release the pressure, this is the reward for them and they go “Aha! So if I just go where she asked me to in the first place, then I won’t have any pressure!”. So, it’s just a matter of teaching horses to use their thinking side rather than their reactive side. It’s a natural born in instinct to react rather than think, but we can certainly make it worse by the way we react to when they react to something. Anyways, sorry for the long explanation, I’m an aspiring horse trainer and I tend to ramble when I talk about this subject matter haha.