I have to ad that I do think there are certainly dangerous horses out there.
Now, I don't know you or your horse, I haven't read any other threads, but I can certainly relate to this thread.
I agree with who said that there is not a cure for every horse, and not every technique will work on every horse.
I owned a mare not long ago, was lied to up & down about her. I really did not mind, however, because I saw a lot of prospect in her, and I've worked with & trained many horses over the years, multitudes of personalities and temperaments.
This mare would do the exact same thing. She would be perfectly fine one moment, and then completely do a 180 and it was as though a switch went off. She would get nasty and take off, bucking, spinning, and running to the walls.
I tired endlessly to work with her, I tried a lot of things. I was never once harsh with her. Still, nothing worked.
My point being, she was just dangerous. I am led to believe that she had something wrong 'upstairs'. She was not completely mentally able to perform a job that I was asking her to. She was the sweetest horse on the ground, sweet natured, but when you asked things of her, I do believe she was not able to mentally handle it.
For what reason, I don't know. Perhaps she was abused, perhaps she had other incidents happen to her previously. But it was not safe for anybody to work with her. There was no correcting it. I ended up taking her to an auction, I was not about to sell her to anybody knowing that she could seriously hurt someone.
So my point, after my novel, is that it is not your fault in any way, and please do not blame yourself in any way. Who knows if she was abused, and who knows what had happened to her before you got her. Some horses simply can not be fixed or altered how we'd like them to be. They are about 10x the size as us, and can be very dangerous animals.
I think you are doing the right thing.
Good luck on your horse search.