Plus the videos on Horsephysio are always mesmerizing!!!
I have same issue, especially to the left. I have been doing lots of circles, serpentines and TRANSITIONS!!! & what Maria said!!! Horsephysio!!!
Trot and canter poles work for balance are great for balance. In this facebook page https://www.facebook.com/horsephysio.ch/?fref=ts you can find a lot of kind of exercises.
Also check your balance, are you leaning, putting more weight on one seat bone, wobbling in the saddle, pulling on her mouth for balance. Check you saddle fit and position, are there any dry spots when you pull tack? Is the saddle interfering with her shoulder movement?
I would recommend the traditional exercises for developing strength, balance, and flexibility are the shoulder-fore, demi-shoulder-in, shoulder-in, haunches-in, haunches-out, and half-pass :)
My horse is having the same problem after having being ridden very infrequently for the past several years before I bought him this June. Transitions, spiraling in and out of circles at the walk and trot and trot poles will help gain balance. Try longing at the canter so she can gain confidence with a rider as well.