Well, this is going to sound counter productive but keep trotting her. Trot trot trot, and don't walk until she starts to relax. If you keep pulling her into a walk, she may keep trotting like that.
Put down a few poles to trot over (spaced out really far) and do lots of figures that don't have a pattern whatso ever. I mean circle, then do a figure eight, then a serpentine pattern, then switch directions, then figure eight, then do 4 circles in the corners, then a half circle to reverse, then more serpentines, etc. Keep her guessing, just make sure you are using your body to ride and turn, not your hands.
If you post, (which I recommend) work on putting more weight down and slowing your posting speed, that will help her out. If you're sitting it, it's easier to get left behind, give mixed signals, or whatnot when the horse is just kind of going instead of listening.
Which style do you ride? What is she doing when you trot her? Is her head up, is it down, is she leaning on you, is she plowing around like a tractor? Is she light on her feet and breathing in short shallow breaths?
"Strength is the ability to use a muscle without tension"