If anger is an explosion, depression is an implosion.
Overwhelming stress has a way of reducing a person's life to almost nothing except the habit and tension of stress. When the stress is no longer sustainable, the person either breaks or falls into depression.
Depression is an emotional state where life seems like an enclosing black hole with no sight of escape. It is quite difficult to overcome depression by willpower alone, but not impossible. The first step should be developing and finding a supporting environment, and the second step should be the willingness and self-discipline to go through an extensive, identity-changing attitude adjustment.
The main problem of depression is lack of vision. By giving up existing limiting perspectives and seeking out new ones, this overall 'vision of life' can be improved.
Stress may lead to depression and the two may coexist for a while after; if they do, a breakdown is more likely to occur in the near future.
'Letting go' is a key concept for both stress and depression.