Erika was hoping that the reversal of the trauma trigger would reverse the trauma and Koto would come to her senses finally, realize she isn't actually interested in dating a child, and be open to Erika at long last. Koto instead has regressed, or seemingly maybe WANTS to regress?
It’s kind of a chilling moment in the previous chapter when that regression is made literal after Aya asks about the seven missing years and Koto replies, “You don’t need to know. Let’s just be together like we were in those days,” and they’re both depicted in their middle school uniforms, not in a flashback but in the present day.
https://dynasty-scans.com/chapters/how_to_break_a_triangle_ch15#20
Koto’s explanation for her refusal to tell Aya what she did while Aya was gone is actually rather nuts: “What am I supposed to say? ‘I wanted to forget how lonely I felt after you were gone . . . So I dated a bunch of people . . .’?”
Well, yeah, that would seem to be a pretty explicable thing to have done when your first love suddenly disappears, apparently forever, and there would be no reason for Aya to hold it against her. But Koto says, “I don’t want her to know . . .this side of me.”
This series certainly has explored new frontiers in “not a solid foundation for a relationship going forward.”