She has her own routine after a mission, away from the rest of the team, away from distractions, away from SHIELD.
The physical location doesn't matter. Sometimes she uses one of the empty rooms in the mansion, other times she'll find a space at SHIELD. More than once she's booked herself into a crappy by-the-hour motel because there was nowhere else. It makes no difference.
She strips off, completely, shedding as much of Black Widow as she can, knowing it will never be enough. She finds a patch of floor, or hard wooden bench, and sits with her legs folded under her, closing her eyes.
Taking deep breaths, centering herself, she begins the repetition. Names, dates, places. Memories of growing up, of her family, of herself. It can take her ten minutes, or it can take her an hour, but she makes sure that it's complete.
Only then can she return to the others for her debriefing and the usual post-mission excitement. But no matter how long it takes her, at least she knows that she is returning as herself.