When a person is feeling depressed and nothing that the Alter Rebbe explained until now is bringing him to joy, then the solution is to break the depression with even more depression.

Because 1. Kach Hi Hamida - this is the only way to break it and 2. This means actually brings a person to a feeling of bitterness which is greater then depression, because depression is lifeless - it's a lack of feeling. However, bitterness is a feeling and the beginning of life/light which removes the darkness of depression.