I agree with everyone else who says that your barn owner is simply wrong in suggesting that groundwork with you will make your horse less aggressive in the field.
My mare has to be turned out alone, as she attacks other horses, but she's perfectly well-mannered to ride and handle and she will not bite or kick when under saddle or being led (unless someone else starts the fight and bites her first). That's about all you can do training-wise. You can't train her to be nice when she's on her own time in the field.
That said, your horse is still very young and very much in the process of learning social skills. You can't teach this, but other horses can. The ideal situation for her would be a stable herd with a strong (but not bullying) lead horse. Such a horse won't be nasty to her, but they won't take her crap, either, and will put her in her place. You may or may not be able to find such a herd, but if you can, you should. Otherwise, she might end up like my horse, unable to have a fieldmates because she's too much of a danger to herself and others, and while necessary, it is quite sad. I know mine is the way she is because, like your filly, she is naturally a dominant, bossy horse. Critically, from age 2 to 7, she only had one equine companion, a rather passive and submissive barnmate whom she could get away with bullying, and consequently she never learned the "proper" etiquette of being an alpha horse and getting on well with her own species in a bigger, more dynamic herd situation.
(BTW, Boulder is my hometown).