What makes it difficult for the poet to forget his love in "Tonight I Can Write"?

The narrator of the poem "Tonight I Can Write" is lamenting a love he has lost—this unnamed but vexing and beautiful woman. In the poem, he clearly states that he held a deep and passionate love for her and that "sometimes, she loved me too". This heartbreaking sentiment implies that his feelings were much stronger for her than hers for him, and she has now left him.
He finds it difficult to forget her and move on from the love because he was so deeply enamored of her, and she is still a strong fixture in his life. In the waning lines, he states that love is short, but forgetting is long—meaning it will take him a long time to get over the love he experienced for her. It seems that he is unable to get over her because he never stopped loving her, in spite of what he says in the final stanza. When she left him, it was a one-sided departure, not a mutual decision, and this has made it all the more difficult to end his feelings for her.