I find quite often I will imagine future conversations with my partner - often these conversations are in relation to something bad, e.g. imagining my responses to her breaking up with me. In this case, I think I see that's some form of rumination; I'm trying to control the fear and uncertainty that I might get broken up with by imagining it to control it.
But I also spend time imagining good scenarios and imagining our conversations and what I would say in them. Is this still rumination? Something else? It's trickier to be aware of it, in a way, because it feels like a positive thing. And yet I can still waste hours of time doing it.