Not sure I understand what you said, but I think in the title is where the problem is: it is NOT keeping the HEEL behind girth with the leg SQUEEZED.
The horse is meant to be ridden 'in position' (flexion to the inside/even bending through the body. That means the hips are parallel to horse's hip and the shoulders of the rider parallel to those of the horse. So legs are extensions of the hips.
That means the inside leg is slightly closer to the girth/inside heel a little lowered/weighted which 'bulks the calf' so when the horse's belly swings against the calf, the horse feels the aid. The outside leg is stretched from hip to heel PASSIVELY slightly down and back (foot still relatively parallel to horse). There should be NO squeezing with the outside leg or pulling it (up/back) into the horse.