Your horse sounds a lot like my first horse. He always had a tendancy to pace rather than trot, especially if he was nervous or unconfident. I totally sympathize with you, even on the One-Speed Trot. Is her trot really really bouncy, too?
The pacing will probably never go "completely" away, but there are a few tricks that might help you encourage her to pick up the trot.
For my horse, "easing" him into the trot was the fastest way to get a pace every time. Instead, to get a trot, I would give a firmer bump with my legs. That would almost always motivate him to skip the pace and bounce him forward into a trot.
Try transitioning over a ground pole. My guy's pace didn't have a lot of suspension, and asking him to lift his feet during the transition was another way to squeeze that trot out of him in a pinch. That isn't much help in a show environment, but it can be a last resort way if she's really stuck in a pacing rut.
Once she is trotting, post the trot. Really do your best to help her find that 2 beat diagonal rhythm. If you can, rub her withers while she's trotting to really emphasize that she's doing a good thing.
Dealing with the speed of the trot wasn't something I was able to accomplish with my gelding, but had I known then what I know now, this is what I would do: Work on suppling exercises at the walk, big time. Once I got to a trot, my horse would go very stiff. More suppleness at the walk lays the foundation to try more quality circles at the trot to build control. Also, once your mare is consistently picking up the trot, use that rhythm to rate her speed. Slow your seat down to encourage her to slow her speed down. Don't just pull the reins to slow her - that'll just cause her to resist and stress, perhaps setting her back to pacing. Try half halting her as well, to rebalance and prepare her to try going at a slower speed. Some lengthenings and shortenings will probably help you gain control of the gait as well.
Good luck! I know that this can be a tough and frustrating thing to work through. Are you planning on competing in jumping? I'm not sure how this works in New Zealand, but if you are going to compete in the Jumper division (In the US, this entails a course of fences, fastest time with fewest faults wins...) your mare's pacing won't be an issue. Not so in other divisions, unfortunately. If your mare is like my gelding was, this will probably be an ongoing project - the good news is, your mare sounds like the issue is not as deeply ingrained as my guy.