I'm confused. Maybe the translation is just bad. In this chapter, it doesn't seem like they have been together for over a decade like it was hinted in first chapter. Maybe translation mistake. I think they've been together for several months or maybe a year but haven't done it until now because she was rejective. Even in this flashback when the girl confessed there was no hint of his girlfriend so she didn't exist back then.
Really confusing with bad translations! You can even tell just reading the bubbles that the translation is bad.