Haha, my horse had the EXACT same problem! She could honestly canter a left circle on a right lead. It was the most uncomfortable thing I had ever felt haha. But, here is what helped me (and trust me! This will take some time.)
Okay, I first gave my horse a que to pick up the right lead. So every time I was cantering to the right, I would turn her head to the inside and squeeze my left leg right on the girth. She was taught with voice commands so all I had to do was kiss and she would canter. But with those two ques and the voice command, she would pick up the right canter. So once I did numerous transitions of cantering to the right like that, what I did was I tried the exact same thing on the left lead going to the left with the opposite foot and turning her head to the inside...she still didn't get it. Haha.
So I would trot her in tight (but not too tight) of circles, and then when she and I both felt comfortable, would try it again. Eventually, she felt so unbalanced of being pulled to the inside, with the outside foot squeeze her and being turned in a circle, that she finally got the que and picked up her left lead. Then! I just kept her going around and around so she felt comfortable cantering on that lead. The next time I asked her, she did it perfectly.
Hope this helps!!!