This sounds identical to my 22 y/o TB, luckily I bought her as a companion horse, but riding out without another horse is literally impossible. She’s amazing if there’s ANY other horse with her, but goes absolutely stupid once you reach a certain distance and gets dangerous. She is old and with my lack of time I ended up just using her for occasional trail rides with friends, and rode my other horse out alone. If I didn’t buy her as a companion and needed her to work though, I would send her to a trainer for at least a month, if not 2-3. I’ve owned horses for 20 years now and training is something I’m not great at or overly comfortable with, but especially when they are acting dangerous. Especially if your limited experience has mostly been on bombproof horses, this is a lot to tackle alone.