@divinityrose I read your story and you said this seems to come from an injury to your neck. Has anyone done an MRI of your cervical spine? I know from my experience as a cervical spine surgery patient that slight changes in the alignment of the spine can cause strange pains in the neck and head, and muscle spasms can cause shifting of the spine's vertebral bodies, the jaw, and how the skull sits at the top of the spine. I have had the occipital headaches you describe, and a tight jaw on one side. What you might want to consider instead of trying to label this as a diagnosis would be to work with a good physical therapist who does myofascial release and cranial work. You may want to see a spine specialists to either rule out a spine issue or find one, and it may be something that physical therapy could help. There may be a loss of normal curvature in your C spine due to muscle spasms. The chest pain and forward head position you describe possibly could be thoracic outlet syndrome. That is often worse on one side and I have that myself. MFR does wonders for TOS and it might be good to explore that even if you don't know specifically what is causing the problem. Posture is very important if you have TOS. You could see a vascular thoracic surgeon for a TOS diagnosis or a neurologist, but look for those specialists at medical centers that list treatment for thoracic outlet syndrome. TOS is misunderstood and not covered well in med schools, so it is difficult to find a doctor that understands it. Scars and surgical scar tissue as well as injuries create restrictions in the fascia that hold our bodies in an out of alignment position. If I raised my arms or turned my head a certain way, my hands would get blue and cold because I was cutting off the circulation because of TOS. There are compression points between the collar bone and rib cage for TOS and it can pull the scapulas forward and to the side rounding your back. My PT has gently adjusted my hyoid bone a few times. I am much improved because of physical therapy.
Here are some links that may be of interest written for physical therapists in detail. There are several on this page for the shoulder, neck and jaw that you'll want to look at including an article on TOS. https://mskneurology.com/category/jaw-head-neck/