A couple of other things to consider are conformation and breeding.
Today's top Western Pleasure horses (as well as HUS, Reining, Western Riding, Trail, and other horses) have the conformation to have naturally low head carriage.
Modern Western horses are 'bred' to have lower head carriage and are bred to be 'slow footed'. These horses are bred to be able to keep their heads down while keeping their shoulders up. The good ones no longer '4-beat' or 'trope'. They keep their shoulders up where they should while their heads are down. 20 or 30 years ago, horses that carried their heads low were on their front ends. The good ones nowadays are not. They can change a lead or a reining horse can slide 30 feet (obviously he's not on his front end) while their heads are below their withers. The breeding and the conformation that comes with this breeding has facilitated low head carriage without losing cadence and quality of movement.
If your horse has a 'ewe neck' or a high set neck, it cannot comfortably carry its head low and maintain a decent quality of movement. If you could post some photos or a video, it would be easier to see where your problems are rooted. The problem can be training or lack of it, or it can be the horse's ability and build.