Horses mirror their rider’s posture. Tension & slumped shoulders create muscle imbalances, causing you to grip & overuse reins. Forcing "shoulders back" rarely works because tension remains. Here is a quick & simple fix to find relaxed, square shoulders & help your horse carry himself in balance.
"If the rider encounters a problem, he must first look for the cause in himself. Most of the time, the rider is the cause of the problem. Only someone who looks at his own riding critically and is willing to improve constantly and learn more will be a good
Your balance and the strength of your core are what allow your position to be unaffected by your horse's changes of speed.
"The quality of a good canter: round, energetic, straight, cadenced, light. You have to feel in the strike off that the horse lifts his forehand without throwing himself forward." ~ Nuno Oliveira
To keep your horse straight in the canter, always be thinking of a slight shoulder fore positioning.