She sure is beautiful! And seems very pleasant in the beginning. Remember, horses are not “bitches” or “assholes”. She learned this reaction somewhere. She did not walk out of her stall one day and decide to be grumpy or have the intent to harm someone. I agree with the poster that mentioned she is turning to the “fight” reaction because in the past the “flight” reaction has not worked.
Correct me if I am wrong, but she seemed to switch her lead behind and cross canter a few times (it was hard for me to tell because I only watched it once and she would go in and out of the frame). She would also lift her head and invert. To me, that indicates discomfort behind. That tight circle is probably very hard for her. Whether it be back pain, hocks, or something else. The side reins made her feel trapped and would prevent her from lifting her head to avoid that pain, so she reacted. She seemed to be trying very hard to please before that. I haven’t read your other threads, does she do this undersaddle or when you are leading her or at any other times besides lunging?
To me, it did not look like you turned your back and she decided to attack. It looked like she went to follow you, you corrected her, and then she reacted. It may have been a result of her previous training. It seemed like she was expecting a yank or smack on her face. Horses don’t like that. Some people will yank or smack, then the horse tries to throw it’s head up to get away and then is yanked again to try and bring the head down. They don’t know what is the right thing and get confused.
I know a horse currently that will react the exact same way if you smack him with your hand, whip, or to harshly with the lead shank anywhere, not just his head. The key with him is to use more body language and less hand and it’s all about making sure he stays out of your space. When I lead him from the pasture we will halt and back up a number of times. The goal being for him to stop when I stop walking. If he does this, we don’t back up and I tell him good boy and gently pet him (he doesn’t like being patted). If he doesn’t, I turn to face him, walk toward him, and expect him to move away from me. If he doesn’t move away I apply pressure with my hand on his chest or gently use pressure from the lead shank on the halter. No slapping, swinging the rope, or loud voice. Also, be slow and definitive in your progression of aids. There is no hurry. As long as he moves away from me I don’t care if it’s a 5 minute process. Quick reactions to these types of horses will set them off. Move slow, make sure they understand what you are asking.
I would wear a helmet. Don’t do liberty for a while if you don’t feel comfortable. Don’t use a whip. Focus on body language and get help from a professional. Also, I only use voice aids if my horse knows the aid. Whoa for slow down, a cluck to speed up, “good boy” as a reward. Excessive voice clouds the other aids and they don’t know what they should be focusing on.
ETA: You people respond quickly here! I was writing my post in between doing stuff so didn’t read some of the later replies before me. Take parts of my post that apply and ignore the parts that you don’t need since you have replied since with updates :).