High Energy Only At Shows

Think of what does change, not what doesn't.

She gets put in a box and stands in a roaring bumping swaying mystery until suddenly unloaded at a place she has either never seen before, or has seen under tense circumstances. There are lots of strange tense horses and strange tense riders moving around her. You too, are unusually revved up, you are not in your usual mood, or wearing your usual clothes. Everything smells different. There are loudspeakers. She knows she will soon have to perform at high intensity, and she dreads how you will inevitably react when she can't control her feelings. Her friends are not there. Her regular patterns and comforts are not there. She wants nothing more than to be at ease but there is nowhere to find it.

I might just be fantasizing, but it sounds like your horse, rather than becoming accustomed to showing, is instead building up a habit of fear and reactivity around it.

If it was me, I would take her to the show grounds when it's empty, and work her quietly there when there's not a lot going on. Then I would take her to shows but not enter any classes, or even ride, just lead her around and let her graze and relax. I'd do both those things until she felt completely relaxed there and trusted that nothing was going to be demanded that she couldn't handle. I would try to set up situations that were like classes but that you could just walk if that's all she could handle that day. Build up her confidence and her ability to be calm, one step at a time.

If you've had her two years, and she's been to forty shows, that's twenty shows a year, which is about every other week, maybe every single weekend in the show season. That's a heck of a lot of stress on a horse.

Horses do need to become show seasoned, but there is aright and wrong way to do it
First, if your horse is not very solid in her comfort zone, she sure is not going to be outside of it.
You a acclimatize your horse to working in different surroundings, before ever taking that horse to a show
There are many ways of doing this, like hauling to different arenas, , just riding around other horses BUT, also making sure you still expect her to listen to you, and focus on you.
You might just haul the horse to some shows at first, walking the hrose around, lunging the horse, and maybe showing in a few in hand classes, and just riding her in warm ups
When I took a young green horse to some of the horse;s first shows, i made s sure to get there early, and I would lunge the horse, then ride the hrose, before the show started, making sure the horse was listening, just like at home.
I then would tie the hrose up, either in a stall, if I had one, or back at the trailer, with food and water] During this time, I would have my own breakfast, and get my show clothes on. A few classes before mine, I would warm my hrose up again
Point is, right from the beginning,you have to make sure your horse understands that the same rules apply in the showring as outside of it, and that is more important then trying to place
Once a horse has gotten to the point yours has, where she is actually rearing,bucking, you have to , first rule out both physical and mental pain, and then take her to a schooling show, where you don't even worry about placing, but rather schooling your horse, while making sure you don't interfere with any other horse and rider
You won't fix as to how she is acting at a show, just riding at home. Right now, by having allowed her to buck and rear at shows, she thinks different rules apply, riding at home, versus at a show
It is only natural that a horse is not going to be as relaxed at a show, as at home, esp in the beginning, that the horse will sense your own tension, which will cause you to ride different then at home, also, esp if you are expecting her to act up, then add the usual show jitters that is just part of showing
But, you can learn to control your own tension, which will in turn , help your horse to relax, and you can make the show ring a pleasant place to be, but, at the moment, with your horse acting up at shows, tot eh degree she does, she needs someone to school her at shows, before you can ever even think about trying to compete

So, to clarify, she does not buck or rear while I am on her. She does it on the lunge line, but she doesn't do that at home. When I am on her, the worst she does is refuse to walk. I stay on until she mellows out, but it generally takes a lot longer than I'd like.

I did take her to several shows before I bought her, and she was fine. She didn't have a show record before I bought her. She does well at 4-H and open shows, which are only 1 day shows. The shows that she acts up at are the longer shows, three or four days. Before those shows, we move in and get the horses acclimated. She follows the same schedule for those shows as she does at home, with the same feeding times. Since we only do two or three classes a day, she generally also has the same riding schedule as she does at home.

She also is generally good after lunging, but through out the day, she gets more and more tense and anxious. I don't beat up on her when she has her melt downs. I just stay on until she stops, and then lunge her more. I was raised that this is letting her "get her jollies out", but at this point, I think it is nervousness, not jollies.

Well, does not matter if you are lunging or or riding her at a show, you must let her know that bucking and rearing is not accepted. You can't expect a horse to do their own thing, lunged, and then expect the horse to behave under saddle.
Forget as to how she goes at home, and focus on how she goes at a show, making her realize same rules apply
Is she ever stalled at home, as she must be at a show?
You might ride her the same amount of time, at home, as in a few classes at a show, but that time at home is in most cases, at one time.The horse then feels work is over, once that ride is-the horse does not measure the amount of time
Therefore, you need to spend some time at home, riding her, putting her back in a stall, then riding her again-same as at a show
If I only have a few classes at a show, I don't just leave my horse in stall, but will ride that horse out, around the grounds, even down a road, if that is possible
I might then just let the horse chill in the hitching ring, relaxing, and put the horse back in the stall and feeding him/her, versus having the horse think that each time he/she is in the hitching ring, the horse has to perform

Further to agreeing with what others have said... If the continual stress mounts to unbearable levels for her, check out the show surrounds in advance, so you know you can take her away to an environment that she can relax and chill at, before returning. Stress builds up when chronic(long term, never ending), until even mild stresses can cause a horse to get to 'the final straw'. But if you can give her even short stress free breaks in between, you can 'reset' her stress levels & then she will handle some more. If you can't do it in a manner that you can do this, then I'd quit doing more than single day shows for some time, until she can become very comfortable with the single days, including being stalled for short spells, and until she's comfortable being stalled overnight at more 'comfortable' places. And taking a *settled* mate with her should help her feel less anxious about it all.