From my own personal experience, the state of chronic depression is a relatively slow & insidious process which begins by the inability to make major choices within a severely conflicting environment. Highly important decisions (those which affect not just yourself, but other people as well) take time to solve. But you dwell on the impact of all future negative consequences and their importance, and these types of situations cannot be solved simply or quickly. You then enter a repetitive negative thought state, constantly dwelling on the major conflicting problems so much so that you can’t even begin to make what would otherwise be very minor choices.
This leads to a complete lack of motivation to actually do anything to help yourself get out of the hole you’ve spiralled down into. It’s a classic example of what physicists term a “feedback loop”. Although you may or may not be aware of it, there appears to be no means of escape -eventually- you become so entrapped by your own pessimistic thoughts that you firmly believe that any choice you make will lead to unhappiness. It’s a terrible state of self pity & guilt. So you begin to contemplate the easiest ways of easing your own suffering, which is of course where the real danger lies. It is a complete breakdown of the mind. Its impossible to snap out of, even for normally hyper-optmistic thinkers.