You are quite right that one can't man handle a horse, but once they have learned to intimidate human, act aggressive towards them, God has helped man invent things like stud shanks and war bridles.
Ideally, a horse never progresses to needing some lessons with these devises, but if that has been let slipped, so the horse has learned he can test a human, you have to use whatever it takes, get the lesson across, make that horse un successful in those vises, ever again, and then, and only then, can real training start.
That is also why trainers would rather have a horse handled very little, then one handled incorrectly
Since this horse became 'spoiled under your watch (not being harsh, just realistic ), I do think you need more help then advise over the internet, at this point in time, and then be set on the correct path, with a horse that has learned some basic acceptable behavior, learning how to keep that respect.
I in no means advocate going to tough measures first, but at the same time, I will use, short term, what it takes to make that horse respectful.
Even when I weaned young hroses, which I did for many years, raising hroses, they were never allowed to turn their butt to me, when I entered that stall. I did not car if it was a defensive reaction, or overtly aggressive- a kick is a kick.
I used a lunge whip, if needed, until they learned to face me. This id not make them fearful, but on the contrary, just set a great ground principle, far as our relationship
A horse could try and pull away from me, once, when I did not expect it. After that, I don't care the age of a hrose,I use a stud shank under that chin, as I am more comfortable using that, then a war bridle, and that horse learns , through physical advantage, which will translate physiologically, that I can indeed control him, and the hrose will then accept that a simple lead shank and halter will control him
Respect comes first, which will have trust follow, and then even affection and a partnership