Dash's entire identity is wrapped up in being the best at a lot of physical tasks, and the majority of those are flight-related. She hates being grounded, even temporarily. So to be told that all her flight-related memories and sense of self never existed would be an immensely crushing blow - one she might not ever recover from.
Anyway I see a breakdown coming. There's always something special in breaking those strong, tough characters because it's making a room for a lot of development and I think this is because there're so many pictures of Dash suffering, crying. Sure it's sad to see any of them crying but when this is somepony that is usually so strong your instinctively think something's wrong and have this urge to hug the poor thing.