You have been in a relationship with Bucky Barnes for a year before the snap that made half the world disappear. During that time, you were helped and helped Steve Rogers helped grieve the loss of Bucky, and end up falling in love. When Tony Stark sacrifices his life to bring back the victims of the snap, aka The Vanished, five years later, Bucky is unaware of how much the world changed around him, including the three year relationship you have with Steve. To Bucky, it had only been a few minutes, but to everyone that remained, it was five, long years. Now Bucky has to cope still being in love with you, but you being in love with another man. His best friend, at that.