Been there done that. I bought a 3yr old that was on the thin side, but supposed to be well trained. Once I got him home, and he started gaining weight, he became a rearing untrained mess. I ended up trading him for a 6 yr old that was quiet and well mannered, and they wanted this youngster for barrel training. When I rode him he was okay, but should have suspected something when he was already saddled when I went to see him. But, once I got him home, he was a nice boy, but still had way to much untapped energy, he was also off the track from Florida. He wasn't the calm quiet horse I wanted. So, started looking for another one and found my blue roan mare, Smokie. She is just what I wanted, 8 yrs old with lots of training and manners. The guy wanted a taller horse, (the six year old was 16.2) for roping and rodeo.... Smokie, at 14.2 was too small for him. This was a trade made in heaven, I love this mare and she is everything I have wanted. I have broke and trained plenty of colts in my time, but decided I wanted a nice calm horse to show and trail ride with, getting too old to get dumped.
When we got Bones, he was over 400 lbs underweight and could barely walk. We brought him home just to rescue him from a horrible situation and figure if nothing else, we would humanely put him down if he was too sick to live or ride. That was almost three years ago, he now weighs in at 1000 lbs and is the calmest horse with no issues I have ever seen.
I am so sorry this horse was not what you wanted. I hope you can either work with her or find her a home and try again. I know what its like to be able to just get on and ride, not worry if the horse is going to explode under you.
I will keep my fingers crossed for you.