Dark Qiviut

Reading the Holiday Special revealed one key detail I overlooked. In her second letter to Twilight, Sunset describes how she feels loved for the first time. Her friends were more than friends. They were family. This comic describes her home life back in Equestria and here. After she finished her letter, she went to have a good night's sleep.

So it's makes total sense why she took her "friends'" sudden disownment of her very hard next morning. And remember, the comic stated Sunset was homeless at the time by having her write to Twilight that night in the library, making their abandonment of her crueler in hindsight.