Half of "The River-Merchant's Wife: A Letter" takes place in the past. The speaker is recalling certain memories about her own experience with marriage in order to express her love for her husband in the fullest sense. As is true in a marriage, not all of her memories are positive. She recalls an awkward meeting, a tough first year of marriage, and then her eventual comfort in marriage. As a whole, though, the past is something that's cherished by the speaker, despite the ups and downs. It's also worth noting that the poem itself is a memory, translated and modified from an eighth-century poem. It's as though Pound were reaching into the Wayback Machine to find something new and relevant for poetry.
The inclusion of negative memories evokes the transformative aspect of the couple's love, in that the husband had to earn the love of the wife. And now she's happy he did!
Memories-shmemories. The speaker uses the past strictly for comfort. By remembering her time with her husband, she forgets his present absence.