Hi Lindi
I'm a little confused by the question
Can she walk, trot and canter in the field when she's on her own?
Or is it just when you are riding her?
How old is she?
I am certainly no expert on this subject by no means - but have you tried working her from the ground instead of riding her?
The only other thing i can suggest is to get the vet out
I did know of a wonderful 16hh Thoroughbred gelding who had trouble with cantering he always felt disunited but he got better with correct schooling, learning to balance and age he was 4 years old and owned by a riding school and unfortunately used in the riding school by kids, people learning to ride - so many different people kicking and hitting him he didn't know what he was supposed to do, i rode him only occasionally but the rest of the time i'd ask him to turn in the stable, pop his lunging 'stuff' on - not on a tight circle but ask with my hand to gently move away from me gradually taking him out further eachtime but the walk and trot are such natural gaits.
Anyway i hope you find out what the problem is - let us know.
Good luck.
Big hugs
Kerry and Carlos.xx
Sorry if i'm a little vague on the situation but