This line is taken from the poem "Remembrance," which is the first clue as to the subject of the poem. Bronte is explaining to her lover how or why her emotions and actions toward his memory have changed.
"Fifteen wild Decembers" - fifteen years - have gone by since he died. At first, she wanted only to join him in "that grave already more than mine." "Tears of useless passion" were shed as she grieved, but time and life moves on.
Now, she has learned "how existence might be cherished; strengthened and fed without the aid of joy." Because she is still alive, she has come to accept that
While the world's tide is bearing me along; Sterner desires and darker hopes beset me, Hopes which obscure but cannot do thee wrong.
She explains that he was, and remains, the "only love" of her life, but time and distance have changed the way in which she carries his memory. She can no longer "indulge in Memory's rapturous pain," but still mourns deeply. Their relationship has been cut by the constant and unyielding force of time, but she still misses him - in a different way.