Hard to say what is going on here. I really think you need a trainer to evaluate this horse, as it sounds like much more than you can deal with on your own.
If it was anything else but the rearing? I'd have advice, but rearing nips any advice in the bud. Not something to try and fix by yourself, and I'm going to be honest here.
I imagine that has a lot to do with her being in need of rescue too.
Whether there are more issues with back, and I am thinking broken bones/cracked bones here? It will take X-rays to rule out many things. And still might never have horse that is safe to ride.
Hi, I've had the vet look at her many times with nothing found, and the vet has said it can only be behavioural....
As regards to a trainer, i've had a friend who breaks and schools horses much like myself, and they have said it's just about showing her there isn't any pain, and getting on a doing it.
She doesn't rear whilst being ridden only mounting, which tells me she's had a pain whilst mounting before, which correlates to the awful saddle she was in when I bought her, now she has had a saddle fitted to her she's a lot more forward going when i'm on, it's just getting on...
I was just wondering because it's more psychological than physical if changing the process would help ease her as it would be a new experience.... rather than something that she's thought about for a long time....
Due to the prolonged period of time she's been doing this the vet has rules out sprains and other soft tissue injuries, the vet has also ruled out kissing spine, and other back issues, and we've had the physio out who has also said in her point of view that there isn't anything wrong with her back, other than her being tense.... so we have done all that is possible to rule out pain.
As you can imagine I was extremely worried about this behaviour as i've worked with many horses including schooling and breaking in horses, so have spend a lot of time and money on medical checks to rule out any pain, and honestly I do think my self capable of training her, I was merely asking for advice and to see if anyone else had done with difficult mounters.