Steve and Tony have been a loving couple for some time, seemingly happy together. Suddenly Bucky comes back in the picture (maybe or maybe not as Soldat first) and Steve leaves Tony cold for Bucky. After some time together (or maybe from the start) Bucky decides he wants to be with Natasha (either a new thing or getting back together) and he leaves Steve cold. Steve decides he loves Tony after all, but Tony, after a time of heartbreak, is now with Stephen and tells Steve to shove it. (Or maybe he has to think about that.)
Like I said, the story I'm searching for may not even be MCU/Avengers, that's just the main fandom I'm reading at the moment. If this seems familiar to someone in any fandom, please let me know. Many thanks.