Spoilers follow. The flash-forwards show Mi-so caring for a child, and the ending explains that mystery. As reviewers describe it, the kiss between Mi-so and Jin-woo is revealed to be imagined, because Mi-so actually rejected him. In a late scene the two women confess that they hate each other. They also reveal that what hurt Ha-eun was the distance between them, more than any betrayal.
In the ending, as viewers read it, the child in the flash-forwards is named Ha-eun. The woman who has been called Mi-so is in fact Ko Ha-eun, who has taken up Mi-so's life of freedom and cares for Mi-so's child. This explains the Letterboxd readings of the title: Ha-eun lives 'like the old Ahn Mi-so' and gives her remaining years to Mi-so's memory. Mi-so appears to have died young, which one reviewer's phrase 'between 27 and forever' hints at. The final image of Ha-eun standing happily on an icy place completes the film's circle. The two women, once mothers to a stray cat, now share the raising of a child. The details here come from audience reviews, so some specifics, such as the cause of Mi-so's death, are not confirmed by the sources I searched.