I had an Arabian mare with a sudden personality change like this many years ago.
She was a wonderful horse, the one I trusted with the 4 year olds. One time my nephew was on her, and saw his older brothers riding my other horses on the other side of the field trotting (he was only allowed to walk). He turned her toward them, dug in his heels, and they began to canter off... I yelled her name, and Whoa! and she stopped immediately. Great horse.
Then my son, much older and very experienced, began having troubles with her. She'd buck, she'd spook, she'd be stubborn, she'd had a complete personality change.
Long story short... she'd developed a benign tumor of the ovary which was causing her to produce testosterone, and lots of it. The hormonal imbalances were making her nuts - she even tried to hump my other mares.
She had surgery to remove the ovary, and went back to her old self.
This was 20 years ago; no doubt they have other options to treat this now.
Your next step should be to have a good vet out to look at her, and discuss looking at her ovaries or hormone levels with him.