What is Major Depressive Disorder? Major Depressive Disorder is a common mental disorder which impacts the way you act, feel and think. It is characterised by persistent sadness and the lack of pleasure/interest. 1 Symptoms of Major Depressive Disorder People may experience different levels of MDD from mild, moderate, to severe. In your mind: Feeling miserable, unworthy and hopeless. Loss of interest in activities once enjoyed. In your body: Exhaustion, trouble sleeping or sleeping too much....