Nine months isn't long in a whiplash injury. The swelling might just be starting to subside. Ironically, that can contribute to instability, and exasperate symptoms.

I think you might really need a through examination of your neck. Have you seen a physical medical specialist or neuro-surgeon? If you don't require surgical stabilization, you might benefit from PT.

The therapist could help you improve your posture and give you simple exercises, like isometrics, to strengthen your neck. Quite a lot of muscle atrophy can occur while you're swollen after the whiplash. When my swelling finally subsided, my chin was on my chest.