Charlie

If it was just a salvia issue, then swigs of water would ameliorate it. 5 months is way too early to "despair". Heck, I've been done surgery for nine months and can't eat and still rely 100% on a PEG tube. If you have not yet had a modified barium swallow test, you should get one. They are the gold standard for identifying what the swallowing difficulty really is as they show the entire process (or lack of it in my case - I have failed four MBS so far)
So get an MBS and keep up with the swallowing exercises. I know just how you feel and it's very understandable and all too real. Still the majority of head and neck cancer patients do regain swallowing and eating. The figure I saw on the OCF pdf is that only 10% need to have a PEG indefinitely. So the odds are with you.
charm