This sounds awful, When you say you seize up- is this you whole body? or just your jaw and facial muscles? If its your whole body I would recommend going back to your docter.
Its a shame your doctor hasnt helped. Recently I have found a few dentists who studied with the British Society of Occulsion Studies (BSOS) who know all about the jaw, bone, teeth and facial muscles. There are a few around the country. Check this link to their website and maybe you can find one near you.
What helped me alot it doing relaxing exercises of the neck and face. Massaging the area that hurts, slowing rolling back my shoulders and making sure im not clenching you teeth or jaw at any time. Its a long and on going process, and sometimes it hurts more after relaxing that it did before, especially the headaches, but you can feel in the different type of pain that its getting better. Sometimes it would make me feel sick - but make sure your drinking plenty of water, this will help your muscles to relax.
Im really sorry you are feeling so bad, I know how debilitating and scary it can be - But when I spoke on the phone to a dentist who might treat me she reassured me that this is actually quite a common problem.
Follow this link and I hope you can find someone who can help:
All the best
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Oh, and also.. Don't chew gum! If you need to to get a taste from your mouth, just chew for 5 minutes and spit it out. And dont purposfully open your mouth to make your jaw crack. I did this, and now my jaw is making more of a crunching sound - I probably did more damage - waiting to find out.