Wow, that is a long time to have the burned tongue feeling. Do you have reflux, or anything of that nature? I understand your desire to pace your self, by talking quieter, but in my experience this can be counter-productive. Muscles respond to working and exercising, and even though they may feel uncomfortable, a little effort can be effective. By taking it too easy, we actually disallow the muscles and tendons to work in their natural way, and we can actually exacerbate any problems.
The jaw becoming, or feeling tired is problematic. First, there is the expectation that it will tire, then the worry when it does tire, and then the subsequent trying to alleviate the fatigue by "holding" back.
A few things that I find effective for any kind of issues relating to the jaw, neck, and larynx are:
-being aware of ones posture. Often people with voice and jaw issues have their head out of alignment.
-not breathing deeply enough. Shallow breathing can be deleterious, as it doesn't provide proper support. Jaw issues aren't just jaw issues, the tongue also plays an important part, it is a huge muscle. Allowing the muscles of the lower back and abdomen can be of great help, as it creates a more even compression in the air, and hence the air is not forced from fatigued muscles.
-and holding back. As I said before, not allowing the body to work things out on it's own. By trying to coddle your voice, you are in effect, creating an intervention that may not serve you in the long run.
I throw this out for what it is worth.
Basso