I don't remember the name, but I know it is on Archive of Our Own (AO3).
In the story, Harry and Draco are hit with a spell that makes them go into an imagination where they live their whole lives as friends and, years later, get married and have kids. However, when the spell is broken, they wake up in reality but can't avoid the feelings that they had during the imagination, so they are still attached to that fake life.
I remember a scene where they go to Malfoy Manor and it is very awkward, since, despite being there for the first time in real life, Harry remembers where everything is because he lived there with Draco in the alternate reality.
Please could you help me find it?