What message is Thomas Hardy trying to convey through the poem "In Tenebris"?

Hardy's main purpose in this poem is to highlight an
unconventional attitude to despair. 'In Tenebris' from Latin means in darkness. He
appears to have reached to a point in life where he is completely desperate. He seems to
be left with no more personal resources. He describes a scene of desperation/desolation.
Due to this desolate environment, Hardy himself seems void of any emotional pain.
However, being a genius of poetry, Hardy turns his condition into an
advantage.

Hardy begins from 'wintertime' which is
apparently the season marking the death anniversary of a passing (death). In his
opinion, winter does not have the potential to bring back the original pain he felt at
the first time of losing his close friend (I suppose his wife). This is why he says
"twice no one dies" - you feel the pain of losing someone only
once.

Next stanza reiterates this sentiment whereby he
claims that he feels consoled because he will not feel acute pain anymore. He somehow
finds comfort. His genuine consolation is that nothing will ever be able to 'harrow' him
to that same extent.

He carries on by stating that the
season is so desolate and depressing that even the darkling thrush 'faint in dread'.
Hardy claims that his strength as well is long gone and that he cannot lose a form of
stamina which he no longer possesses. Ironically, to him this is an
achievement.

He explores the theme of emotional destitution
(leaves turn to dun). 

All in all, what hardy aims at is
stating that through the passage of time and the events throughout his life (the death
and loss of all his close ones) he fears nothing. He is prepared for death and stands
waiting. In a way, he even challenges nature and death because he believes that he has
nothing else to lose and nobody else who will be hurt by his
death.