I happened to have depression before the cancer. I'm sure it contributed to the decline of my physical heath and the eventual cancer as I didn't care enough to look after myself - nothing really mattered at the time and I didn't know what to do about it.
If you even have an inkling that there is something wrong with the way you are feeling, or you are not just right in your thinking - get help.
I went to a pain clinic during/after my treatments and a neurologist thought there was something more he could do for me. He saved my life by referring me to a psychologist and a psychiatrist at the Tom Baker Cancer center here in Calgary.
I was diagnosed with bipolar depression.
Along with talk therapy, I have since tried many different meds. I'm now on Mirtazapine and it has lifted my spirits and helped me gain weight! (It's not for everyone - YMMV)
In my experience, if I had to choose between cancer and depression, I would choose cancer. I still say that. Depression can be even more deadly. Get help and do not think this is a minor thing that may go away in time, as it's not worth the risk.
BTW - eating Lindt Red Chili Dark Chocolate brought back my saliva after 3 years of terrible dry mouth. Or at least I am telling myself that.