With my young horses to start with I always bend their head to the outside and trot on the wrong diagonal and push them into canter until they are more balanced and have better understanding of your aids. Repetition is key 😊
Ask for canter in corners or circles and over exaggerate your aid, and remember to look where you're going for a few strides before checking because if she is on the wrong lead and there's always a down trans straight after that could make her difficult to keep going in the canter. Once her transitions into canter are becoming more consistent slowly decrease the exageration
If you try trotting in a small circle to the right and easing into the canter there, it is very difficult to miss the right lead, because the horse's inside shoulder is dropping in during a circle, causing them to pick up the right lead. It is always easier to get a lead out of a turn.
My horse had the same problem, and the way we got him cantering was very simple, i jumped a very small fence and that way he always got the canter right i rewarded him. I know thats not a usual way but it worked...
My horse did that. What worked is sometimes doing a simple change across the arena or ask on a circle. Other wise Make sure you sit through the transition, wieght in your inside leg... ask with inside leg, opening inside rien (lift up more) keep your contact in your outside rien.