Lateral flexion. Also, start out with smaller circles and use lots of leg with a more open hand.. but keep the hand pulled back out towards your hip.
For lateral flexion, don't give up. My mare's first attempt was us going around in a tiny circle for a good five minutes before she finally got the idea. Try doing it ten times for each side. Ask slowly but give immediately. I noticed a difference within a couple days.
When you're circling, wait to see the neck muscles contract, then release and change direction.
Forever loved, never forgotten; my beautiful Indie. <3 Hoofprints on my heart.