you could always try a rehabilitation centre most of them will take her on.

Why not advertise her and see? Make sure you fully describe your horse and state that she's for free, to a good home only as you don't want her to be pts somewhere else, unless a vet says it is needed. And where are you based?

I agree that you need to check for a physical cause of this behaviour, perhaps even get an equine physio to look at her.  Before trying to find her a new home, is there any way you could send her to a reputable person for reschooling?  It would need to be someone who is understanding of behavioural issues though as conventional schooling methods would only make her worse.  I hope you find a solution to the problem though, it's a shame you've not been able to enjoy her.  I once had a pony who was very stubborn and badly behaved and someone suggested to me that he should be pts, I ignored her and worked away with him, he just was really spoiled in his last home and literally would get away with murder, he just needed time, patience and understanding and I had him for 12 years.  Good luck,