Then please explain this. My horse likes to canter on the straight sides of an arena with his haunches to the inside. I can correct this by using my inside leg at the girth along with asking his shoulder to come to the inside in shoulder-fore. However, when I ask for a medium canter he tries to fish-tail to the inside again, losing all straightness and he gets tense and his head comes up. I can ask for a half-halt with my seat to reposition him but he just gets more tense and it's a wreck.
So I put the double bridle on. I am able to get him more collected in general with the help of the double bridle and when I go to ask for a medium canter I can straighten his body and he does not fish-tail around and boom! A great medium canter which I can actually bring back into a collected canter without having to apply a strong rein aid just to get him listening to my seat? Why is this happening? What is so majorly wrong with "putting another bit" in his mouth? He obviously respects the double bridle and I have more control over him. He will learn to be more respectful of me and my seat aid.