Well, here's a small positive thought:
I read somewhere that when judgeing a horse to buy, you should base your yes /no based on how good the horse's canter is, because it's very hard to change or improve a horse's canter, but it's much easier to imrove ahorse's trot.
More than likely she is used to moving in a certain hollowed out way, and though she is sound, it could be that the saddle doesn't fit so well. Or, she just is built in such a way that it'll be a hard trot to ride.
Is she perchance very wide in the chest? Post a video .
And yes, getting her to round up would help, but it's nowhere near as simple as just putting her in a snaffle and riding or lunging in a snaffle. Just out of curiousity, what bit are you riding her in now?