**Disclaimer: I am NOT a pro-trainer, so take anything I say with a grain of salt and your own knowledge of your horse.
** I have never hit or had to whip this horse**
If you ask me, you're going to have to start. That behaviour is EXTREMELY rude and disrespectful, and 100% unacceptable. Not to mention downright dangerous. My horses know better than to even threaten to nip or bite. If they do, they get a loud "Hey!", and the one time my gelding actually did nip me, he got smacked HARD on the neck and I've never had a problem with that since.
You have to start saying NO, and meaning it! Your horse has crossed the line from threatening to hurt you to actually going through with it.
I know it's never pleasant to have to physically reprimand a horse, and if it came down to a contest of strength and aggressiveness, the horse would win every time. But it's not about that, it's never about hitting your horse. It's about being more dominant than your horse. You have to mentally hit harder than your horse can bite, charge, kick, stamp, or whatever. You have to win the mental battle. When you smack your horse, you're not doing it out of anger or for "payback". You're smacking, or reprimanding, in order to say "That is NOT okay and I will not accept this behaviour. Do it again and you will be disciplined again."
Of course, there is a line that separates discipline from punishment. I don't like punishment. Punishment can often go way too far and escalate to abuse. Discipline has to be swift, firm, and only related to the behaviour that warrants it. If my horse bites me, they get a "Hey!" and a smack at the same time, and then I'm done. I don't have to go any further than that. If my mare starts acting buddy sour on the way to the arena, she gets a tap with the crop and then I am done until she acts up again. Never do more than necessary, and sometimes that can be hard to distinguish.
It takes time, patience, and forgiveness. And all of that takes discipline of yourself, before you can properly discipline your horse. It's a process that we are all in different stages of learning, and I would STRONGLY recommend getting a professional out and maybe even shadowing a trainer.
As far as what you need to do specifically, that is up to you. I would go back to the round-pen and work on demanding respect from her. You have to be EXPLOSIVE if she charges you, be loud, yell, jump around, make her uncomfortable and respectfully fearful of what will happen if she decides to be dangerous. Don't be afraid to hit her with the carrot stick. That stick won't hurt her no matter how hard you hit (just never her face), but if she runs into you? That's not a risk worth taking.
I would also suggest doing so in SHORT sessions. No more than ten minutes, and fewer if she shows progress. The second she backs off and starts acting respectfully towards you, call it a day and let her be a horse. Or, spend a short session in the pen before each ride, every other day. Do things to keep it interesting, like putting a pole in the pen that she has to trot over or something like that.
It really depends on your and your horse. I'm just spitting out ideas, but it all depends on what you feel comfortable with doing and what your horse will respond the best to.
If you can't get a trainer out to help you, keep researching. It does sound like you're a bit in over your head. As another poster said, this isn't Heartland and you're not Amy Fleming. At this point, your safety has been compromised with this horse and you really need to change your methods if this is how your horse has been behaving. It's not her, it's you, and you are the one who needs to change before she can.
So, don't give up. Research, get help in some form, get a second opinion, get someone with more experience out there and learn everything you can about this horse, how she responds to situations and where her buttons are.
You got this, just go slow and steady and keep reaching out for advice.