It's interesting that this only happens at extended trot. Has anyone checked her hindlimb abductor muscles, her forelimb extensors, or for other physical issues that could be causing this? Is she particularly expressive in extended trot? The reason I ask is because something like this:
is not a classically correct extended trot. The cannon bone of the hindlimb should run parallel with the radius (upper leg) of forelimb, such as:
If your mare does the former, it's possible that she is over compensating for her lack of balance.
If we have ruled out a physical problem, then it could just be a bad habit. Having a narrow shoot made out of poles may help her to refocus on her back legs and place them closer together. If this helps, they could then be placed on top of trotting poles in order to encourage her to step better. I would definitely rule out a physical problem first though, particularly if she isn't conformationally base wide.
Hi Phoebe 🙂 working on gymnastic with small cavalettis could help you! Your horse will have to be more under to do the job right. Transition work is also the key
Hope it helps