Haven't finished the story yet, but reading about your experience with the post-main quest line, it's exactly how i've felt when i did the same in The Witcher 3, pre Blood and Wine. Difference is, in that game, i think after the main story, most of the established NPC's were also gone, so it felt that much more emptier. Regardless, it's something that happens whenever we finish anything truly touching, like a good book or tv series. It's a testament to it's greatness.. that ability to linger on in our thoughts to such a degree.