Yeah, I'm surprised the NTI device didn't work for you, just based on the fact that you cannot no matter what clench your jaw if its in your mouth correctly. The splint helps me in my case because my bite is very uneven - my molars on the left don't touch when I bite down, which is why the left side of my head always hurts. The splint creates an even bite plane so that the pressure of clenching is evenly distributed throughout my jaw, not just one one spot.
Ask your dentist about
getting a splint made, with insurance it will be affordable. Also, visit a neurologist so they can evaluate your headaches and possibly prescribe a muscle relaxant like Zanaflex. I actually take Klonopin (a benzodiazepine used for anxiety) every night to suppress the urge to clench at night. I think addressing your stress and anxiety is the best way to prevent the clenching. Good luck :)
"You cannot hope to build a better world without improving the individuals. To that end, each of us must work for our own improvement and, at the same time, share a general responsibility for all humanity, our particular duty being to aid those to whom we think we can be most useful." - Marie Curie
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