It sounds like he hasnt developed the balance without it yet. Does he give it to you at all without it on? If not, you used it too much and he is resisting. If so, then stop using it and just keep asking. Repitition develops muscling, muscling is needed for balance. Balance is needed for consistency. He just isnt there yet.
I use things similar to this, but never for a whole session. I use them more as translators. Once the horse understands what I am asking for, I don't need it anymore. They are littlerally only used for maybe 10 minutes a session for maybe 3 sessions. Then they are worthless.
Yes Delete he is indeed 18, and has been set in his way of going for a long time. This is why it MAY be appropriate to look for a little help in the form of artificial aides to try and make the changes.
You can see from Mr Gilmores pics there how far we have come without any outside assistance, but a year on, even though we have built new muscle he still prefers to carry his head in the old way. Working with draw reins once a week has helped us to take another step forward.
dressage is a set of building blocks where you start with the most basic and work your way up. I'm not opposed to a skilled person using certain training aids on certain horses in certain instances like some "anti training aid people" are. But if your horse is falling apart without the martingale, my guess is he's not going as properly as you think he is. There is a building block missing from your foundation.
From the sound of your posts I think you're trying to ride the front end of the horse instead of the back end. For a horse to be balanced, collected, muscled, on the bit, etc, it's all about the horse's back end. See-sawing is never correct in dressage. In fact, I don't even worry about my horse's headset. I work the whole horse and somehow the 'frame' just comes together. If your horse is head bobbing my guess is that with the german he's probably avoiding the bit (giving to pressure, he has no choice, but more avoiding rather then accepting) and probably a little on the forehand (does he support himself with your hands instead of having self carriage?). When you take the german off I have a feeling he's avoiding, just by going up with his head this time. Our goal is to have a horse with self carriage, who can keep going correctly even if you let go. I'd probably work a bit without the german for a while and concentrate more on his lateral work.
I will agree that my lack of knowledge in training any form of an english horse is the problem.
We aren't looking to do any sort of competitive dressage, it's just a pickup event for HS Equestrian Team, a lot of kids do it without having ever ridden english before in their lives, which makes me feel better about the whole situation...But of course I want to help Rebel and myself to at least have a shot in placing, because things tend to go wrong and if we can get any other events to participate in, it will help our team and ourselves...
Also, do you think I should ONLY be riding him in his D ring, or can I change him back to his gaming bit every now and then? I figure even when we are doing our gaming and western events I can still work on drawing him back up into his position, during the warmup and cooling down periods and sometime in between turning the barrels.