My experience was similar-- no energy for at least 3 months. I had tongue cancer- surgery, reconstruction, then radiation - 32 treatments I think it was. That was almost 10 years ago. I had a recurrence last spring, with more surgery. Got my energy back a little sooner, with no radiation this time!
Even after 4 or 5 months, you can expect good days and bad days- no rhyme nor reason to the bad days. I'm just happy I'm still here, on this side of the grass! You're not doing anything wrong--just keep eating as healthy as you can, exercise (maybe walking ) a little every day if you can, and sleep when you need to! You'll need lots of sleep to help get rid of the after-effects of all that radiation!