Thanks, that's what I've been trying to do. Problem is, my father is rushing for a sale. So he rides her and not only is he asking her to jump, he's very unbalanced. I mean he can stay in the saddle just fine, but he has very little balance going over jumps and even just working on the flat. That could be causing tension.
I haven't found her over flexing with me too much because I'm not trying to do much. I am afraid she's gonna buck so I just sit calmly while she does her stuff around the arena. But I have no control. So if I ask her to go up a pace she will either move too forward, start prepping for a buck, or if I have contact on the reins she will overflex.
It can be difficult -- though not always impossible -- to help a horse learn to relax while being ridden when others are riding the horse without this goal in mind. The fact that your father is rushing for a sale probably conveys itself to the horse in the form of tension which would cause corresponding tension in the horse.
When you are riding this horse at a walk, try to feel whether the horse's muscles are tense or relaxed. Work to get that relaxed feeling before even considering a trot. Breath deeply, talk softly, hum, or even sing softly to give the horse assurance that everything is all right. Think of calming an anxious child.
Try to establish just enough contact with the reins so you can feel a different with the bit if you slightly flex your fingers. Try to keep this contact constant as your hands follow the movements of your horse's head.
Be sure your body is moving with your horse. At the walk, you should feel your seat bones moving in small circles -- down, forward, up, down, forward, up -- about 180 degrees off from one another. When you think you feel your horse's muscles have become relaxed, test this by stopping all movement in your body. If your horse has truly relaxed and become accustomed to your body moving with hers, she should stop when she feels you stop. If she doesn't, begin moving with her again working at relaxation before you try again.
Once you have established such relaxation at the walk, you may try whispering for a trot. This may just be a slight flex and release of your calf muscles or simply a soft verbal "trot".
I recently worked with two young girls who were having trouble getting a quiet trot from their horses. The horses appeared to take off quickly and the girls began pulling on the reins. I told them both to put their hands on their horses' necks and keep them there when they asked for a trot. Both horses began a much more relaxed trot.
I hope these thoughts provide you something to work with.