I find longe line training is one of the ways to fit a horse.
As well as the other suggestions I would try this.
Using a surcingle, you can tie your horses head to the right when longeing right, and tie it left when longeing left.
You just bit your horse in a D or O ring snaffle, and tie the head with leather reins. (Leather is breakable.) Start out by tying the horses head little by little. Start out loose, and by the end of the week or month depending on the horse, you can tighten it up and really bend the neck.
If your saddle has rings, you can tie it there instead of using a surcingle.
This stretches your horses neck, and then in turn, the head will drop naturally.
Hope that helps!