Psychiatry separates unusual mood states from the usual run of upsets. The latter are reactive depressions, generally down to lack of support from broken companionship commitments.
Affective illnesses are called endogenous depression (endogenous means not caused by events from without but arising from changes within), manic depression, puerperal depression, unipolar where the affect is on the one side or the other; bipolar where it shifts between both.
Depressive Affect illnesses are extensive and global, lowering every interest and energy, including enjoyment of food, sex and hobbies, the things of the day. It is not focussed on one particular thing though one aspect may be isolated.
Changes of mood
Where the mood is heightened there is excess which often takes the form of surprising energy; little need for sleep; promiscuity, spending, irritable dominance.
People who suffer from depression can be experienced to attitudes such as: 'pull yourself together ... do something positive ... make a change ... take a holiday '.
Using such ordinary explanations in the lives of the people afflicted can be very frustrating as this does not help.
If you are finding that your mood is constantly low and you are finding that you are struggling with general living it may be an idea or see your GP or to look for a therapist.