Depression. Classical picture is one of chronic sadness, loss of interest in people/activies, fatigue/tiredness, excessive sleep, poor appetite or eating too excess, emotional lability (crying easily over anything). Some people, however, present with anger & resentment. When depression is severe, some may have thoughts of suicide. Your pcp can screen for depression & rx meds that may help + psychotherapy.
Lack of energy... The classic symptoms of depression are decreased energy, decreased motivation, decreased interest in pleasurable activities, and decreased self esteem. Some people eat too much and some people hardly eat at all, because they say food just doesn't taste good anymore. If it lasts for more than 2 weeks or you feel life is not worth living you need therapy, medication, or both.