She does have an old injury from racing that doesn't bother her at all normally, ...I'm ordering the back on track mesh sheet as well (possibly saddle pad too), because she does have a long back so is prone to back soreness.
Very impressed that it sounds like you have covered many bases trying to rule out physical probs.
As the people you have consulted all seem to agree, there's a fair chance they're right too. But then what you have written above appears to contradict that. What are you feeling the need for that mesh for? When is she 'prone to soreness'? What is the old injury(never met an OT horse without...) & when does it bother her?
As you have said about the mesh(I'd be very hesitant in using something that promotes even more heat under a saddle BTW), I take it that muscle stiffness is a problem. Lack of magnesium particularly, sodium &/or too much potassium, among other nutritional possibilities could be behind that & even if there is no 'real' problem that a bodyworker can find, tight muscles of themselves can cause discomfort, esp in exercise, which may not show up as obvious pain when the horse isn't working.
While all these 'experts' have apparently agreed, it is also worth keeping in mind that horse chiropractics and bodywork is an unregulated industry, so they may or may not be well educated, skilled people.
When I ride I'll go, lunge her to warm her up for 10-20 minutes, then I'll get on and do lots of circles and long and low walking. I'll ask her for a trot and she'll instantly pin her ears and buck. She will buck hard, if I punish her for the buck using my crop or dressage whip she'll buck higher and sometimes rear. She throws complete temper tantrums.
That sounds like a lot of circles, especially for an ex-racer. This is hard on bones & joints. I do appreciate that some like to lunge in circles for exercise, but for a variety of reasons I'd be ground driving, taking for walks, doing hill work, etc, instead of so many circles.
Have your bodyworkers checked her & her saddle out while you've been trotting? Saddle/rider pressure is higher at a trot than a walk, and studies with pressure sensor pads have shown that pressure points from imperfect saddle fit, rider balance, etc, are tripled at the canter as at walk & trot.
Doesn't sound like 'temper tantrum' is likely, but if not pain, then fear, memory of pain, etc. Horses are very associative thinkers & previous experiences don't necessarily fade without effort. Punishing a horse for pain or fear responses will indeed cause it to escalate, because it's just adding to the 'badness' association. Has your 'coach' or other experienced rider ridden her? Does she behave any differently with different people?
Today she trotted a couple laps, but that's it. Cantering? Don't even think about it.
That's great & assuming it's not pain/discomfort, but fear/association, if you managed to get that & stop *before* it got too much for her. You want to practice as much as possible, to get better at, the 'right' emotion/behaviour, while minimising likelihood of her practicing the 'wrong' emotions & reactions. Therefore if you reckon you can get a couple of laps out of her before she 'blows', I'd be just asking for only one for now. Lots of walking, interspersed with single laps(or whatever you think she can give well) of trotting should allow you to get better at trotting & replace her 'bad' associations with good ones.
her attitude comes out and she doesn't like the indoor arena (which is my only option in the winter),
So perhaps the fear association is with being ridden in the indoor?
I'm thinking natural horsemanship may work? But I wouldn't even know where to start. Obviously I should get a rope halter and long lead line, but where from there?
If you don't know anything about 'NH' then I wouldn't worry. Excepting to learn more about equine behaviour, bodylanguage, behaviourist training theories, which will of course help. If you have a good instructor/trainer to work with, who is skilled, considerate of the horse & observant, whether 'NH' or otherwise should be neither here nor there. You also don't need a rope halter to do 'NH' & you must already have a long lead if you lunge & ground drive??
Can't find it in training forum(tho I see you posted a same thread there - best to stick to one & not duplicate), but there was a recent post about bucking at the trot & one handy piece of kit - which won't change the *reason, emotions, association* but will prevent really bad bucking & keep the rider safer, while you work on retraining, is Cherie's 'overcheck' type string.