At 3 weeks post treatment, you are right at the point where you should start to feel better. I'm sorry to say the dry mouth lasts for awhile, ask your doctor for something like Salagen to help that out. You can also try Biotene products that are made to help with the dry mouth. Look at Walgreens or any pharmacy.
My appetite didnt start to come back until I got rid of my feeding tube (at about 4 weeks out). I'm now 2 months out of treatment for tonsillar cancer ( 3 chemo, 7 weeks radiation). Your doc should also be able to give you something to help you sleep. As for the depression and anger, I cant help much, I get depressed about it too occasionally.
But....it does get better, just takes some time.
Good luck, and Merry Christmas.