I have veered from shock to numbness to anxiety through the first half of active treatment. I handled surgery pretty well, and I walked off my anxiety every morning over 5km and often came home to update my blog or rant on this site about something. Chemo gradually knocked the physical stuffing out of me which made it impossible to do those walks by about a third of the way through. Depression and a feeling of pointlessness has come by to visit and like an unwelcome visitor, refuses to move on. I am trying (unsuccessfully) at the moment, to book into a psychologist - I don't know if I will be able to open up to her/him but I need to try as this is a long haul.
What you have said about earlier depression helping with this experience has rung some bells for me. My older sister, who lost her fight with BC in 1999, had also suffered with mental illness for all of her life and started receiving treatment as a teenager. As I was younger, the family policy was to keep me in the dark about a lot of stuff that was going on, but I do remember her saying on a couple of occasions, that she was very comfortable discussing the treatment side of the cancer (but not the emotional side). I wonder if it was because it was something that for her, seemed tangible? My sister's mental health always caused significant problems for her but my admiration for how she lived with the BC has grown with my own diagnosis.