You can really see how Mitsue got ground down over the decades in this chapter. Compare the outburst of anger and feelings of betrayal when she finds out about the marriage in this chapter to the barely whispered "do you remember our promise" from after the divorce and you can see just how much she's changed over the years.
And it looks like Kiyoko has been ground down over the years as well.
I think we can understand why Kiyoko still means so much to Mitsue even after all these decades, their time together in high school completely changed Mitsue’s life, and even after all the decades she still had the same love for Kiyoko as before.
Thanks for the depression as always