Lots of good advice - I'll add my own personal anecdote.

I, too, have a hot curler = ) He came to me with preexisting bridle fears (was either through the bridle or behind it), but he presented a very pretty picture like your mare. Luckily my instructor at the time immediately recognized his hind end was not engaged. We did a lot of stretchy circles. On a circle, I'd really make him bend to the inside and make him use that inside hind and then let him relax, then ask again, then relax, etc etc. All the time allowing and encouraging him to stretch down with my hands. We also made him slow his tempo during this exercise so he couldn't run his trot. I did a lot of this at the walk at first to help him get the idea, also at a pretty slow tempo. Then we did it at canter because the natural rhythm of the canter was easier for me to get in a good ask/give/ask/give groove. Once he really discovered the stretching, he learned to like it quite a bit (though he's still a tad more BTV while stretching than would be "perfect") and it was a great tool to get him using his back and hind end properly. That, in turn, helped him develop a much more consistent tempo and added a lot of expression to his gaits. You could feel his whole body unlock and his front and back end start working together instead of being separate entities.

Shoulder-ins at the walk on a circle also helped. I did a clinic with Sally O'Connor, and we did these from the ground too (she describes the exercise nicely in her book, Common Sense Dressage.) It helped show him the way to the contact, he'd start seeking it out.

What also helped my horse immensely was upping the difficulty = ) As we started introducing the more interesting stuff, he got FAR more engaged in his work. He loves the collected and lateral work and has also really started to figure out his lengthenings. Giving him a lot more to think about made him a much happier horse.

Pretty much now the only time I have a curled up mouthy horse is when we're walking not doing anything and he's bored.

As others mentioned, and you certainly know, your riding and position has a huge impact too. My guy is very compact and sensitive, so if I'm even a little unbalanced, it shows. If I get just a touch forward, he'll drop his shoulders (and used to immediately curl up again). It's wonderful for learning (if I do it right, he does it every time, so I know immediately when I'm doing something wrong, haha), but it can be frustrating = )

Lastly, I also found he vastly prefers a baucher bit. He is a very busy mouthed horse, and he seems to like the stability of the baucher. A loose ring was a failure for him - too much to play with.