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.
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.