For me the dividing line is in how productive the thought is. If the thought leads to new understanding and behavioral change, I'd call neither regret nor worry

For me the dividing line is in how productive the thought is. If the thought leads to new understanding and behavioral change, I'd call neither regret nor worry obsessive. That said, there's a bit of a definitional trick being played here by most people. In the just-released "The Tomorrow War", one character is portrayed as irritatingly obsessive about a science topic. Their detailed knowledge turns out to be useful later, at which point respect for him is portrayed. The judgment criteria shift in retrospect, even though the behavior is the same. A darker example comes from "Maus", a memoir about a Jew who barely escaped Nazi Germany. His pre-escape behavior in abstract could easily be described as obsessively worried. But in practice, the ones who were more "reasonable" went into the ovens. My takeaway from this is that apparently maladaptive behavior is often just adaptive to a different environment. That's certainly my experience.