This is the same for depression as it is for chronic pain. We can survive anything... if we had an end date. Severe pain that we Know is temporary? We can handle that. With no end date? That is difficult to come to terms with and cope with. Endlessly cope with I should say. And some days we do not cope well. Some days we do.
The thing about chronic pain though is that There Is No End Date. It is fact. And that fact can enhance our depression if we suffer from it. I have depression associated with chronic pain. So the pain is high and I am depressed... and the fact the pain is endless is a point that occurs to me often in that state of mind.
Because depression in-itself without chronic pain is insidious, and compounding, and it shrouds the future in its own view of reality... so you cannot see an end to it. You see your suffering lasting forever because that is the twisted logic depression gets you into. You are locked in this fog without an escape.
Add them together and you get a very firm fact, your physical pain as not end date, and turn that into.... your suffering will never end. Depression is never 'There will be good days. There will be bad days. But you will cope like you have every other day.' No, depression is 'you entire future is consumed by suffering and pain, like your entire past was consumed. What kind of life is that?' There is a logic to depression, but it is a special sort of logic that exists without a shred of hope for the future. It leaves no room for potential. And change. It assumes the future will be like the past... but the fallacy in this is that it assumes it will be like the worst of the past, and ignores that a) you had low pain days b) that you coped up to that point c) that you survived all the hard times in your past and d) every damn good thing in there... and just focuses on the eternal pain stretched before you. That is why it is insidious... it twists things and tricks you into looking at things in a specific and narrow way.