I just want to go on record here. I was a hasty voter and did not read for comprehension when voting on this poll many moons ago. I DO NOT believe full bridles are a teaching aid, and I blame my laziness and love of polls for premature and thoughtless voting. (I also almost voted for Lyndon LaRouche for President back in the 80's...if you know who I'm talking about you're showing your age
My position is this. Double bridles are used in a correct dressage program as a refining tool and not as a teaching aid. The poll choices themselves show OP's lack of understanding and experience in upper level dressage, and frankly, correct dressage overall.
That said, I can see OP's situation in that she finds what seems to be more control of her older, incorrectly trained horse when she uses the DB. Many roads lead to Rome, and Rome in this case, for me anyways, is OP feeling...something different, and learning from it. We dressagefolks don't all have access to talented and experienced horses and trainers, and from the horse's description, he has been ridden incorrectly for eons and is now in his old age being asked to do things his body isn't willing or able to do well or easily any longer. I wonder about soundness/soreness issues, having maintained my old GP horse in competitive form into his 20's. Is he getting joint supp's, injections, chiro, massage, even some bute before or after a tough day?
When a supposedly 3rdL 19 y o horse is unable to remain straight I would look to soreness/misalighnment and rider error. Using a DB gave OP more control over the body, but the DB should never be used as a solution. If OP is truly committed to the classical goals of dressage she must understand how critical it is to follow the basic goals of dressage-relaxation, straightness, thoroughness, etc, at every gait in a simple snaffle.
And if this horse at his advanced age is unable to offer thoroughness, straightness, relaxation while forward, I'd exhaust all efforts to see that his evasions don't scream OUCH. And if he still evades after correct and sympathetic riding and bodywork, I would consider a carreer less strenuous for the old guy.
Just 2 cents from an old(er) dressagecrone