Could be a saddle issue, just had to say that.
Have you tried just not worrying about her lead,but just let her canter? Sometimes they have to learn how to balance themselves, and letting them do the wrong thing helps them learn on their own. It's harder to canter with the wrong lead in small circles. So for now,just get her going, and steer her in small circles. It will be ugly at first,but it works. THey get tired and many times will want to change over on their own, especially if they get the correct lead without a rider. I also find that once YOU stop worrying about the lead, so does the horse. If you are overly concerned, then the rider will start inadvertantly sending confusing signals. Just relax, go back to square one, and she'll get it. When she finally gets it, just go a few strides, enough to enforce you did want her to canter, then let her stop or slow down and rest. Get 3 or 4 good lope offs and stop drilling on it for that day. Each day you can let her go longer, and she will be building muscle and stamina and balance on that lead. It won't take long before it just won't matter.
Keep cueing her the same as you always did, just don't bring her back down immediately if she flubs up, at least initially. And dont' worry about steering her in a small circle when she gets going. Let her lope off before you start spiraling in, and let her make bigger circles when she's on the correct lead, making the "right" thing easier, the wrong, harder.