Ask your horse to move their shoulders. Pick a diagonal line in the arena and try to move ONLY their shoulder over and keep their head and haunches behind. It'll teach them to bend their rib cage and the base of their neck.
Surprentines, circles, and figure 8s are great exercises.
Do lots of flexing your horses head and do Lots of turns with your horse and you can get a good bridle for head strong horses.
Okay this might seem weird but
What I do is I have a good connection with my outside rein. Making sure my horse is pressing into the bit slightly. Inside leg at the girth and then pull up(not back) on the inside rein. Or more up than back. This makes her want to round out and bend to the inside. If you don't have a contact with outside rein then they are bound to just follow where you are pulling them.
I usually just get a treat in my hand and make her stretch her whole neck side to side following the treat in the stable before working
Work with her bending her neck on the ground then start working with her on the saddle....
Do the bending exercises with veggie treats.