I'm a bookseller and manager, not a psychologist, but I can tell you that recurring nightmares are super common and rarely anything serious. What usually happens is your brain picked up on some anxiety-inducing scenario somewhere and it keeps replaying it because it's turned into some kind of automatic pattern. The fact that you've been having it since 2015-2016, that's a long time, but it doesn't necessarily mean it's rooted in something really deep. Sometimes it's just a loop that's set in and keeps going round and round.
What bugs me a bit is when you say it's become a "real obsession" and you talk about it with everyone. So maybe the question isn't so much "why am I having this nightmare?" but rather "why am I obsessed with this nightmare?" Because once you start talking about it a lot, thinking about it before bed, waiting for it to happen, it reinforces the whole thing. It's like a self-fulfilling prophecy. And honestly, if it's been eating at you this much for a decade, it might be worth seeing a therapist or psychologist, not because something's wrong with you, but just to get you out of this loop that seems to be weighing on you pretty hard.