I don't think she purposefully kept quiet, I think he went wandering and she thought she'd never see him again, so she gave kotaro the letters in hopes he could find him. At least that's how I interpreted it, though it could've been that there was another reason she kept quiet, maybe because she felt guilty for having taken up so much of his life-though that feels like maybe a partial reason but not the full story.
She couldn't find him bc he kept travelling. She wrote letters but she kept them all since she had no address to send them to. She stayed in that house for decade waiting for him to come back. Also, she didn’t know wether he got married to another woman in the meanwhile. I'm glad they are gonna spend the rest of their lives together
Bruh, the ending makes zero sense. There’s no valid reason for her to have kept quiet about the fact she was still alive for DECADES. I’m soooooo pissed!!