I'd be interested to check if tack is an issue, does everything fit ok?
My horse is a big dude (belgian draft) and had absolutely no respect for my personal space when I first got him. I would be giving him a pat and all of a sudden he'd whip his head round and I'd get clobbered - he paid no attention to me whatsoever. The good people of NR recommended groundwork and watching that he didn't move my feet - that I moved his. Simple things like just getting her to back up on your command might help instil some manners.
We're very much still a work in progress and he has his moments of craziness (his favourite intimidation technique is to bomb around the pen to try and force me to let him out into the field.) He has cantered straight at me - not angrily, just childish 'I want out mum' play and I now stand my ground and say 'NO!' (Although I'm quaking in my boots I won't back down easily!)
They can be extremely subtle about their tactics in moving you around so watch out for that - the headbutting is very clear, but the little moves they do to put you where they want all add up to less respect. I would be leading Flipo out and he'd all of a sudden started nudging up against me and before I knew it I was leading him from the other side. Took me a while to realise he was doing these sorts of things on purpose. That flamin head is so big it doesn't take much for him to push me around.
I also had a huge problem with the farrier when I got him - he needed sedating as he would just not play ball and was far too strong to fight with. Then along came a trimmer who has been able to work without any drugs at all - anytime Flip decides he's had enough the trimmer just calmly but assertively backs him up and its like he turns into a little lamb. (I started using this technique for our mounting problem - he wouldn't stand still, and he still protests but only once).
Lift your energy, be firm (but not nasty!!), if she won't get out of your space then feel free to get big - wave your hands around. Do some groundwork and think of yourself as bigger than her. The book of NR is 101 Horsemanship Exercises, well worth a read. Good luck! And would love to see some photos of her!!