Imagine being involved in a terrifying incident where you were physically harmed or threatened. Then imagine reliving that awful memory over and over again, each time as fresh and horrific as when it happened. This is the essence of PTSD.
In overcoming depression, one of the main areas you may need to change is the way you view life. For example, if you believe that life consistently treats you unfairly, then the inevitable ups and downs of life are filtered through that perception.