Interesting to hear you think you're ready for a grand prix prospect when you can't put your current mare on the bit? My belief is that you have to pay your dues with horses before you get a really special one. You can pay all the money in the world for a horse with amazing potential, but that doesn't mean you will be able to ride it. Stick an average-jo rider on Totilas, yeah he's currently the top horse in the world but I would put money on it that they're not going to be able to get him going anywhere near what a highly experienced rider would.
My opinion - stick at it with your current horse, don't waste a good horse when you can't put this horse on the bit. Sorry, harsh but true.
How are you trying to 'put her on the bit'... you say she'll 'go on the bit' at halt - that sends alarm bells off to me that you're either jiggling/sea sawing or pulling back to get her to drop her head. Also at halt their not on the bit as it's not a forward motion and they can't in effect, reach into the bridle.
I am also highly curious to see your 'collection/extension, changes and counter counter' as I am a little dubious that a horse that cannot/will not bend around the inside leg and give through the back, neck, poll and jaw is able to perform these movements to a satisfactory standard.
My feeling is that you are far over estimating the extent of training involved in reaching grand prix. By all means, may Grand Prix be your goal. But do not lose sight of correct and basic training. A horse may well be able to 'fake' most of the movements up to grand prix.. but its likely that the horse will break down early, mentally or physically, without the basic training established first.
You need to investigate and follow the German training scale, or pyramid of training - Google Image Result for http://www.dressage-academy.com/images/dressage-training-pyramid.jpg
As you can see, collection is at the peak of the triangle, it can only truly be achieved when all proceeding training has been firmly installed, with a horse that is happily swinging through the back, engaged through the hind quarters, stretching from the wither, relaxed in the neck, poll and jaw, and willing to move straight, forward and in rhythm.
Higher level movements are achieved once collection has been established. Yes, basic, first steps of those higher movements may be trained to assist in the gaining of collection, but they will not be performed with any type of quality until the horse is in true collection.
I don't know what kind of 'fantastic riders and trainers' have been riding your mare, but I'd suggest going to someone who has the qualifications/results/etc. to ride your horse or teach you. If you and these 'fantastic riders and trainers' cannot get her 'on the bit', yet can teach her changes, extentions etc. I would be thoroughly worried that they are teaching entirely the wrong thing.