I would not jump to the conclusion to fire your trainer. None of us are there, none of us see what exactly is going on.
I have had students whose horses had developed habits over years of doing things without balance, or strength, or suppleness. When you begin unraveling those issues, the habits unfortunately still often remain. You can believe that it is ok to take years to sort them out or you can believe that you should use some sort of auxillary device to show the horse there is a different way to do what they are currently doing out of habit so the horse can change as soon as possible. Retraining is so different than starting from scratch! So depending on the situation, maybe the German Martingale was not the worst idea… I don’t know, I am not there. I probably would get on the horse myself, figure out exactly where the hole is, work on that and then IF I thought a device was needed, use it myself to correct the issue/break the habit and then return the horse to the student. Of course that is in the ideal world where students can afford to put their horse in training or partial training or at least pay for 6 training rides in a row (for example).
Phew, I sure hate the time it takes to type out a disclaimer!
So after all that, I think DressageArt has given you excellent advice.
The only thing I might add other than don’t necessarily ditch your trainer, is that some horses really need you to just throw your inside rein away in the transition. IF you can do this (if your hands are that independent of your body) I would say this might be enough of a difference for your horse to rethink what may have become a habit. Sooooo many horses get pulled on, on the inside during transitions, that I find this to be pretty effective in breaking the cycle when retraining. I think they get into their hissy fit because the inside rein being pulled on stops them from using their neck to balance. Most horses HATE to be off balance (and the ones who don’t have no self-preservation, no thank you - it’s not a bad thing!) and when you refuse to take the inside they realize that they are going to be allowed to use their neck to balance after all. And the brouhaha stops.
So that may be worth a try, if you can do it.
I did ride a horse once that I was warned that if I took my inside rein during the canter depart that he would throw me. Ok! Maybe that is why I am so quick to let it go when things go wrong… :lol: