Have you had the horses teeth floated and made sure the wolf teeth have been pulled? Horse starting at the age of 2 until they are 5 years should have their teeth done yearly as they are continue to shed caps or baby teeth until they are five. After their 5 year old float they can normally go on an every other year program as long as motorized tools are used.

80-90% of young horses who throw their heads or hold them up high is due to dental issues. Most horses should have their first float before a bit is ever put in their mouths. I had a client whose horse had to be done every 6 months for the first five years, he was just soo sensitive to the sharp points, that he became a bear to ride and as soon as his teeth were done he was pack to his sweet self.

But once a horse has learned to push through a bit, fling their heads, or what ever else they might do. IT will take them a little while to figure out that is doesn't hurt, when you ask something of them and it should get better. But their are those horses that once they learn something it can be very hard to break them of it.