I had severe TMJ problems early on. I had the locked up jaw like Kristara mentioned.

Two things helped.
Neurontin (gabapentin) reduces my jaw clenching, especially during sleep.
My PT has been able to do some myofacial release to allow my jaw to relax and move back into place.
I have seen two TMJ dentists with little help. It is not possible to get the appliance into a locked up jaw. When I could get the appliance in, I could not stand it, let alone sleep. It caused a gag reflex.

There is a Botox procedure where they inject Botox into the TMJ muscle to weaken it temporarily so the jaw relaxes and can get settled. I have a bump on my lower jaw line where the tendons of the TMJ muscle inserts into the lower jaw. This bump is caused by the stress from clenching. If you have this, it is evidence of the damage the clenching is causing.

It is difficult to get insurance coverage for TMJ unless you can directly connect it to an injury.

Hope this helps.

My best to all you other jaw clenchers.