My horse keeps breaking to the trot in counter-canter through corners — how do I fix this?

Breaking in the counter-canter corner is almost always a strength and balance issue rather than a disobedience issue. The horse simply does not yet have the muscular control to maintain the lead through the centrifugal demand of a corner on the false lead. The fix is to back the exercise up to a level where the horse succeeds, build confidence and strength there, and slowly increase difficulty. If full corners are too much, work only on the straight portions of the arena in counter-canter and avoid corners entirely until the gait is secure on straight lines. Then introduce very large, gradual corners — think of barely tracking a serpentine rather than a sharp turn. As the corner becomes easier, gradually tighten the arc over weeks and months, never asking for more than the horse can hold with relaxation. Strengthen the horse's hindquarters in general with transitions, shoulder-in, travers, and trot-to-canter departs — all of which build the same muscular base required for counter-canter balance. Also check your own position in the corner: collapsing your weight to the inside or twisting your shoulders unintentionally makes the horse's job significantly harder. Ride the corner as if it were a straight line — quiet, centered, following the horse's motion.