You should try to change exercises when you notice he is maybe getting bored or not paying attention. Like if you are doing work on the track, when you sense his/her concentration is leaving, do cercles, a figure 8, a halt, a change of gait or any other dressage figure you feel like doing. Stay concentrated on the horse too.
The horse I ride gets distracted and spooked but a lot of things, so my trainer makes me do figures than come back on the track. Sometimes just halt and change direction and continue your exercise. Also, I play in my fingers so he knows he has to work with me. It's just to keep a presence in his mouth. Hope that helps. xxx
Change it up all the time. As you are going along the fence line at what ever speed and do random circles or figure 8's. Trails help a lot as well!
Work on circles, transitions, spirals, leg yields, figure 8's and go out on a trail ride every once and a while and if you can go cross country or ride in more than one arena if possible. He's probably just bored!
Change it up. Stay relaxed but I know that when I am doing the same things over and over I get distracted too! My guy sometimes needs a canter in the middle of working on walk/trot transitions. Helps keep his mind in the game. And remember to have fun. Work hard but don't let every time you go outside be work.
I think that if it's a relaxing moment to her, you should respect that and let her relaxing ... Outside is often used for the horse's mental, and with the ups and downs which are muscling your horse without a particular effort appart from walking ... You should just share a good moment without working with your horse if she really doesn't want to, and work at home :)
Try to relax him. Maybe begin with circles and stretching. You need a lot of patience and being satisfied of little things ... :)
Transitions to keep him listening to you. Spiral in and out on a 20m to 10m circle to keep his attention. Change up the pattern of movements you go through.