Warning: What I am about to say may seem counter to much of what you’ve heard from other physical therapists, trainers, etc… No matter if you have headaches or neck pain or back pain, DON’T try to correct your posture, especially if it hurts to do so.
Our bodies naturally position themselves to take pressure off of nerves, because nerves don’t like to be squished or pulled on. Not at all, and they will let you know immediately when you are doing so. Often people have a lateral shift with a backache or a forward head with neck pain or headaches. This is your body’s way of protecting the nerves. If you artificially correct your posture to be “straight” or “correct,” you’ll be irritating sensitive structures that are already ticked off. Kind of like picking at a scab (yucky but true): You’re not doing anything permanently harmful, but you are slowing the healing process.
So, what should you do? Move, gently, and in ways that feel ok, even if that means only small movements. Your purpose is not to stretch tight muscles (as this is actually not even possible), it is to get your muscles to relax and soften, cultivating a feeling of ease. Which, ironically, is not easy.