I'm trying to find this Bucky fic.
Basically, the protagonist (reader or OC I can't remember) works for the Avengers but has a secret she never shared with them. The Avengers and her are summoned by Doctor Strange to fight in the mirror realm when she and Bucky both grab the time stone at the same time and get sent back to 1940's before the war starts.
She befriends Steve and Bucky from back then. She tells Steve to pick her up every morning as a message for him to carry to the future to come find them.
She and Bucky fall in love slowly as she begins falling for pre-war Bucky and post-winter soldier Bucky at the same time.
I think pre-serum Steve likes her too.
Every time Steve comes to get her she refers to Bucky as "The Man Upstairs".
They stay with Jarvis and his wife.
Slowly Bucky begins to get flash memories of what she's creating that help him through his time as the winter soldier and make his time at Hydra less terrible.