Can someone who has experience with this help?

Every.single.morning I wake up with a really dry mouth, a symptom of mouth breathing. It's been the same all my life.  

So I've recently tried a 'sleeping mask' (see graphic below). It keeps the mouth closed and the teeth together. However, I haven't been able to finish a night with it on. It makes my jaw clench and I end up with a really tender masseter muscle

Is there anything else you've tried that keeps the jaw close enough together that the tongue is, at least somewhat, on the palate during sleep?

I'm not talking about 'mouth taping' because, as far as I can tell, that just keeps your mouth shut but doesn't prevent the jaw itself from slacking off - the result being that your tongue still sits in the floor of your mouth

Any ideas?