Sorry to hear that. Some are too much. It hurts bad when you can't help them. I've done rescue work all my life. It hurts when you have to put an animal down.
I had a mare I wanted so bad, but she had almost killed her owner. Mostly due to his stupidity. Boy was she gorgeous. I must admit I was drawn by her spirit and the challenge. I worked with her for months, but I'm not the most experienced. Even with guidance from a trainer we couldn't bring her out of it. My friend, the trainer refused to let me buy her for my own good. She said as bullheaded as I am, I would end up dead. She is probably right.
She is wild now, her owner took her home after almost 6 months of recovering. He doesn't do anything with her.Not the best decision, but it was his to make.