I am a teacher--of music, specifically choir, voice, and general music. I expect things of my voice that few would.
I do have a slight lisp. I gather that it sounds worse to me than what it really is. I do not have the lisp when I sing. It does not hurt when I talk, and I talk or sing a lot during school hours. I have directed variety shows and musicals since getting out of treatment. But not while I was in treatment, which is where you are, correct? I think that would have been impossible for me to do. I just did not feel well enough, and I was fatigued, and when one is fatigued, it shows up in the voice. That may be why you have pain, especially after talking for hours.

At the end of radiation I tested out my singing voice and found that I had "big chunks" of pitches missing, or so it seemed, but that rapidly improved. I already had excellent technique which I had learned years ago and had repeatedly practiced.

You could certainly see a speech pathologist and get some exercises that could improve things for you. Good breathing technique can help you. I did find out recently that my larynx looks good, unharmed(videostroboscopy) even if my thyroid has taken a hit.

Best,
Anne