As a sufferer on and off over the years, I've done some reading around the subject and have found that the scientists seem to believe that some depression is linked to specific, activating events or episodes...even somebody using a particular word. Spike Milligan - a notorious depressive - said on many occasions that what led to his debilitating periods of depression in which he could not leave his room was usually something somebody had said. For Churchill too, depression was episodic. He referred to it as his little black dog.
For others, depression seems to be unrelated to an event or time but just arrives unlooked for.
For those people who suffer with the first type of illness that I describe, Cognitive Behavioural Therapy has a good track record of helping. The approach involves naming the activating event or word or whatever and either avoiding it or somehow changing the perception of the sufferer. Takes a while and a lot of effort but I can vouch for the fact that it works.
And one theme seems to shine out from the research: that sufferers are more likely to recover of they take the conscious choice to feel or act differently. You've heard the joke about "How many psychiatrists does it take to change a lightbulb? Answer: one but the lightbulb has to want to change." The jest hints at the truth....you're more likely to recover if you choose not to suffer alone and unacknowledged. By taking the step of posting on here, or speaking to your GP or asking a friend for help or calling Samaritans, you've made the choice to survive. Very big step.
If the forum will allow me, I can show this by telling a story that some on here will know: the story of Viktor Frankl. (Apologies if already been posted). Viktor was an inmate of a Nazi concentration camp in WW2. All his family except for him perished. He suffered unspeakable cruelty and deprivation.
Later, he wrote that he survived solely because he chose to exercise the only freedom he had left....to choose to survive. He noticed that some prisoners kind of gave up and accepted the situation. They got ill and died quite quickly.
So: in his head, he spoke to his wife. He'd been a lecturer at a university before the war so he gave whole lectures to his students....in his head. And he survived....in fact the more he did it, the stronger he got. By the time that the camp was liberated, he was supporting and keeping alive a whole group of prisoners and even some guards by his strength and force of will.
All by choosing to survive. All sufferers have that choice. It's not a privilege or an entitlement. It's yours because you are human.
All the best to all those who suffer. You are most definitely not alone.