I'm sorry to hear about your cancer diagnosis and subsequent treatment. I finished my treatment two and half months ago and have my PET scan in 2 weeks. While the pain I went through was bad it never got so severe that I needed fentanyl (I used low dose morphine). I managed to eat throughout my treatment, though towards the end I was on soft foods. But it would take up to an hour to eat sometimes because of the pain. I used to use oral lidocaine before eating to lessen the pain, but it did not always work well enough.

I would advise you consume enough calories (and protein to help with healing) every day. Stay hydrated too.