I am better circling to the left, too. This is because my right side is stronger. When I circle left, my outside (right) hand and leg act in a 'supporting' manner. The left hand acts in an 'active' manner, and this is little more than a soft 'tickle' of the rein to create an inside bend (to the left).
The opposite direction is less balanced for me.
I've also read that most horses are better going left. This is thought to be because of how the fetus is often bent that way, in the womb. And, my mount, X, prefers me to be on a right diagonal. that means that if I am traveling left, the 'wall' is on his right side, and I am rising and falling with the leg on the wall , the right leg. Even though we really only trail ride, whenever we pick up a trot on a straightaway, he invariably wants me rising with his right fore.
Or course, we watch the fore leg, and match it to our posting diagonal only because we can SEE it. What REALLY influences posting is the REAR leg. We are lifting off when the rear, outside leg is pushing off. This is why we change our posting diagonals to match the OUTSIDE rear leg's thrust.