The disorder is depression.
Explanation:
The continuous feeling of sadness or loss of interest, inability to face a new day, changes in sleep and appetite are the symptoms of depression. In the given scenario, Diane seems to have depression where she shows loss of interest, hopelessness, sadness, mood swings, irritability, restlessness and also has put on weight.
To treat depression, one should go to a counsellor and share what they are going through to lighten their burden. One should also involve themselves in some fun activities. One must not rely on medications only. Exercise regularly and ensure to have a proper sleep.