In what way is the poem "The Darkling Thrush" an attempt by the poet to search for meaning in the world?

The speaker of the poem is worried about the dawning of a new era. He's comfortable with all the old certainties of the 19th century, and he doesn't feel particularly enthusiastic about what lies ahead. The song of the darkling thrush acts as a reminder of what he's about to lose—yet it also stands as a potential source of hope that may fortify the speaker during his difficult transition from one century to the next.
The speaker ends the poem by candidly confessing that he was unaware of the blessed hope held out by the thrush's song. This would appear to suggest that he is throughly pessimistic about the future, believing in his heart of hearts that the new era will denude his life of all meaning, that it will undermine all those things that have given his life some sense of purpose and direction. That being the case, he will have to construct meaning for himself, as people are wont to do in times of personal crisis. Any such meaning will doubtless be constructed out of the sweetness of the thrush's song, and all the joys and beauties of nature it represents.
https://www.poetryfoundation.org/poems/44325/the-darkling-thrush

The speaker's state of mind in this poem is described as "fervourless," or lacking in any kind of enthusiasm for the world around him. He sees the wintry world around him as reflective of this state of mind: it seems to him "shrunken," and Hardy makes an allusion to the anxiety that struck at the turn of the twentieth century when he refers to seeing "the Century's corpse" in the lay of the land. He is clearly feeling uneasy, unsure of himself and the world around him, and is seeking some hope when all he can hear is a "death-lament."
The voice of the thrush, then, goes some way towards providing this for him. Its "joy illimited" is very different to what the speaker is feeling. Indeed, he notes that there is "little cause for carolings" in the world he sees; he cannot understand what the bird is experiencing that he is not. And yet, the song of this thrush does give the speaker "some blessed Hope." While he himself is still "unaware" of what that hope might be, the song of the bird seems to spread some happiness into the desolate landscape and gives the speaker a shred of hope to cling to: he may not be able to see the cause of the thrush's happiness, but he does believe that it is there somewhere in the world.