Erin, I also assume that you have checked out teeth and health possibilities that could contribute to your mare’s tendency to rush. Horses can become bored with the same ridden routine. I would plan out several exercises and figure combinations that would cause the horse to have to listen as they do not know what you are going to ask of them. Have you tried using problem solving skills to keep your horse thinking of how to find the good deal. When horses are ridden in a boring routine without the rider having an ongoing conversation with the horse on what they want, expect the rhythm/gait to look like and the tempo/collected, medium or strong energy should feel like, the horse tends to become the leader and takes over the decision making. As she seems to favor the canter, have you tried cantering earlier on in your ride? She sounds like a horse who needs to move her feet. Longing is boring in the long run. It can allow a horse to burn off steam as they run, buck and fart on the circle, but as the horse doesn’t feel and purpose to this or intent from the rider, it doesn’t relax the horse.
I would add in some Parelli or other Natural Horsemanship to build a relationship with the horse. I use Parelli 7 games and other exercises to connect the horse to me before riding and then build my work on what I have done on the ground. Lateral work/suppling is so important to having a straight horse who is on the aides. Cavelletti asks the horse to think. Having caveletti set for a more collected trot, working trot and another set for a bit of lengthening ask the horse to think and solve the problem of how to do the rails in a comfortable relaxed way. Having other cavelletti that are raised to 6″, 12″, 18″ and eventually make sets of varying heights. All these things keep the horse interested and thing of how to solve the problems. There are several books on cavelletti and exercises to keep your horse interested. “Creative Dressage Schooling: 55 Imaginative Exercises to Avoid Boredom and Enjoy the Training Process” Hardcover – August 12, 2014
by Julia Kohl (Author), “A Gymnastic Riding System Using Mind, Body, and Spirit”, “The Problem Horse Repair Manual
A Toolbox for Today’s Rider: Ways to Overcome Bad Habits and Problem Behaviors Under Saddle “Doug Payne. There is a new book on cavelletti and cones. A grate book,” Caveletti” by Klimt
I have used the seven basic Parelli games with hunter/jumper students and dressage students who were amazed at how much better their horses listened while ridden.