She sounds exactly like my warmblood. Or how he used to be anyway. When I got him he was labeled a biter and a kicker, would charge at you, strike bite and rear when given shots and was said to be dangerous, vicious and unpredictable. He was going to be put down soon because no one could handle him. He is a very dominant horse with people and if you don't know what you're doing, you could get in a situation that not in your best interest. That hasn't happened to me, because I knew what to do going into this, but I can see the potential for things to get dicey with someone who doesn't know as much. This issue with this filly needs to be addressed NOW before she gets any bigger. I delt with this issue with my warmblood only 10 months ago and he's 16.2hh.
It's so sad that people force horses to go to this place because of their ignorance. Horses aren't born with this nature, they are forced to it by people's bad handling. But it's so rewarding when you can help the horse take down those defensive walls and earn that horse's trust and respect.
This is going to be a challenge for you, that's for darn sure. But you sound very motivated to help her so I applaud you for that.
I'm not sure if you are familiar with Parelli but that's what I do with my warmblood. It's made a world of difference, as I knew it would. I would highly recommend you look into that. But here are the key things I worked on A LOT with my warmblood to establish my space and to get him to respect me more.
First of all I wanted to teach him to stay away from me. First and foremost! So I had my rope halter and a 22 ft. Line on him (because 1, he's a big horse, and 2 because I wanted him WAY out away from me). I started teaching him one of the Games Parelli teaches, called the Yo-Yo game. I got that to where he would back away from me with a pretty light asking. Then, still using the idea behind this game, I would walk away from him while he followed, stop, turn and back him up immediately. This is a safety drill. Could I get him away from me in a hurry? How long did it take him to tune in and be a little more sure of where I stood?
Then I worked on getting him to move away from me with steady pressure. Could I get him to back up/yield to my hands? It is SO important with these horses that you ALWAYS start fair. Rub them first, then start asking them to move with pressure like a feather. Here are the phases I go through- press the hair, then the skin, then the muscle, then the bone. I do this VERY slowly so the horse can link each phase together. You must remember to reward the very slightest try, otherwise you could make these horses want to fight you. These horses are not afraid of people, they will fight if they feel threatened or challenged, so that's why you can't bully a horse like this. Because if you try to, they will win. Hands down. You should never bully a horse anyway.
Always remember to start off as softly as possible. She might be desensitized to it now, but if you work with her enough she will become sensitive to the lightness again. But you have to offer it to her in a way that she will respect and trust and not feel challenged by. Basically I did, and am still doing, Parelli with my warmblood, so that's where my success with him has come from. It's hard to really explain over the computer, especially with these kinds of horses because they require your complete, undivided attention and you have to read them constantly. It's a very delicate balance when it comes to getting our point across....too little and the horse won't feel our leadership and take advantage, too much and the horse will fight. How can you get her to respect and trust you in a way that is palitable for her? Will she look at you and offer you the world or will she look at you and want to be somewhere else? I think this little filly will change you if you go about this right. My warmblood has changed me and the way I look at things now, he keeps me honest with my emotions and is not afraid to tell me if I screw up. So good luck with her! This is a wonderful opportunity for you to learn A LOT so try to enjoy every second of what she will teach you.