In Episode 23, Diana finally learns about the Dream Fuel Spirit. She had this reaction which was a combination of shock and rage before she brushes it off to focus on learning what happened to Akko.
Diana reacted that way not only because she learned her idol, Shiny Chariot stole her magic but also because of the consequences that came with it.
Since many of the Cavendish household had learned about Diana's condition, we can assumed that they may have help the girl recover. The one who would be most invested would be Diana's own mother, Bernadette.
The most sad possibility is that before Diana could regain what was a good portion of her abilities, Bernadette passed away. Bernadette might have died in regret, believing she failed her daughter. Meanwhile, Diana is distraught that she was unable to show the beautiful magic she could use to her mother.
When learning that Chariot didn't help Akko when she disappeared, Diana feared the worst. Not only was she out there all alone, Akko was alone and dying in the cold and filled with despair that her dream was destroyed. The exact same way Bernadette went through.
Diana: I'm disappointed in you. Not what you were back then but what you are now.
To Diana, the greatest sin anyone could ever done is let someone die in regret.