You may be suffering from a condition called chronic whiplash syndrome (CWS) or chronic whiplash disorder (CWD). Patients with CWS/CWD report chronic pain, loss of motion, and disability.
There may also be a fear of movement or reinjury. This can cause a patient to become overalert or hypervigilant about pain and movement. If you think extending or rotating your neck is going to hurt, you stiffen up to avoid pain or discomfort.
A combined program of physical therapy (PT) and cognitive behavioral therapy (CBT) may help. The program can help restore full (painfree) motion, strength, and function. You will be given cues to breathe, relax, and focus throughout motion.
At first you have to tell yourself to use these techniques whenever you start to feel threatened by motion or certain positions. Over time, you should be able to retrain yourself to move freely without thinking about it.