I agree this is a spoiled filly.
Experience eventually teaches you how to read signs in horses quickly, what horse you can give a treat to, which one you cannot, what their intentions are and how to treat them accordingly. Say you have a timid sensitive colt who is the type to get scared over anything and everything, and he in turn takes a lot of desensitizing and kindness. There are so many different personalities.
Then you have the type who would like to be spoiled. They are meaner natured, less worried and more confident, with a bossy streak. Those colts are never pets. It takes a lot of handling and getting them broke before they can be treated as a pet, and some never can be.
That said, any horse, especially young horse, who treats me with aggression is going to be treated right back the same way, only bigger and meaner than he started. Of course, the nervous nelly takes the correction of a small correction and believes it is big. The punishment fits the crime, but also the personality.
I am not good at that with humans, but many are, so you can relate it to that. You react to humans based upon what they need, your boundaries are upheld to the level of the personality you are dealing with. Horses, in my opinion, are just as complex as that.
Now, I have a filly currently who would have liked to have acted like what you have. Her nature is mean spirited and she’s a bully, and I love her! However, she didn’t get to be a pet. I didn’t let anyone handle her who didn’t know how to, because they’d have gotten hurt and ruined my filly. My boundaries were clear.
Yet, whenever I felt her going in that direction, I took her for a trip down to the round corral and did much of what @Zimalia22
mentioned, although I simply think of it as running a horse in the round corral. Rarely did we make the trip (because I don’t have one at my house), but on occasion I ponied her down the road to where one is, and she remembered I could make her move, and thereby was the boss of her.
I am not exactly sure what I would do had she been spoiled, because I had full intentions of not letting her become so, because as you can see it is a big problem once it begins. I am afraid you are going to have to be very clear in your lines and intentions, and it’s going to take being aggressive right back, yet still having excellent release and not being mean to simply be mean. I think that will take someone else, simply because you seem to have lacked the experience to read the end result of small things. This person is going to need to be good, but a good person is rarely going to want to deal with something spoiled. Once you find that person, and make sure they are good because bad training will ruin any chance you have, tell them the situation. They can deal with the horse and then watch you, and make an honest opinion of your capability of handling the animal. If they say you can’t, let them sell her for you on commission.