Please note that this topic is given more extensive treatment in Dr Nicholas Ray's detailed profile of Freud's essay Trauer und Melancholie.
Freud contends that the difference between mourning and melancholia is that in the former all that has been lost has been lost from consciousness and there is a necessarily painful withdrawal from what has been lost, but in the latter, melancholia, it is not clear what has been lost because the identification has involved unconscious components. Whatever was taken in from the deceased was taken into the unconscious, and its loss provokes a loss of self-regard and a sense that it is the ego which is empty, not the external …
Clark, Robert. "Mourning and Melancholia". The Literary Encyclopedia. First published 22 October 2005
[https://www.litencyc.com/php/stopics.php?rec=true&UID=1606, accessed 21 February 2019.]