My old mare was a hand reared foal that I bought as a 3 year old to save her from being put down (my vet, also their vet, who had been asked to euthanise her asked me if I would give her a chance) She was a 15.2 monster because she'd never been taught boundaries and discipline, she had been allowed as a foal to 'jump up' like a dog and put her hooves on their shoulders - imagine a 15.2 that still thinks they can do that, they were shoving candies down her every 5 minutes to placate her which was like a reward for bad behaviour. She'd never been kept with other horses so first thing we did was to take her away from her human herd and put her in with a firm but safe old gelding who taught her in about 5 minutes that her behaviour wasn't going to be tolerated by him. Second thing I did was to hit her really hard and she knew from that moment on that I meant business and 'no more monster'
This filly is just going to get worse if she isn't leaned on really hard now.
Could she not be split from the herd she's so clingy too and kept with one sensible horse? Maybe kept in a stall situation with a daily turnout paddock where she has to rely on humans for her needs instead of relying on the herd.
You're not in a great situation here but a horse with this sort of attitude is not for the nervous and /or inexperienced owner. Someone stands to get seriously injured.