Agree with faye.
This all hindsight of course, but I think a couple of things were missed with this filly. A horse that refuses to work in the ring but that will go on a hack is telling you something - she's telling you that the ring work is a.) boring b.) painful or c.) both.
When I looked at the top photo, my first thought was "That's an awfully demanding frame for a 3 year old." And she's already showing some evasion - a little behind the bit and bent at the third vertebra not at the poll. I wonder if when you hacked her out you let her go on loose or long reins or if she was expected to be in a frame then too?
Some horses have the body for a job, but not the mind, some have the mind but not the body, some fortunate ones have both. This filly may just not have the mind to do the kind of work you want her to do, or at the very least, didn't have the mind yet at three years old.
Also agree with faye's suggestion to send her away for retraining. I would give her a three month break with turnout first and start over slowly, carefuly and without putting a lot of demands on her. And she may be permanently sour to the kind of drilling in the ring that's expected of a dressage or event horse.
It's a shame, because she looks to be a lovely, athletic thing. Good luck bringing her back.