Squaring my horse

So im working with a new horse for halter shows and or showmanship and i am having the worst time teaching him how to square up, ive never had an issue before with this with my other 9 horses but i also taught them at a young age, this horse is atleast 7 and is as stuburn as a donkey, any suggestions im at my wits end and it prolly dsnt help im starting to get fustrated, help, please???