Why did Helen call the frost king incident a dark 'cloud' in her childhood's bright sky ? How did Helen emerge stronger from the experience

Helen Keller was accused of plagiarizing her short story called "The Frost King." She had written and sent it to her mentor, Dr. Agnagnos of the Perkins Institute for the Blind, as a birthday gift. He was initially delighted and published it in a Perkins Institute report.
However, it was soon discovered that the story was very close to a tale called "The Frost Fairies," by Margaret Canby, that was published before Keller was born. Keller and Miss Sullivan at first were able to convince Mr. Agnagnos that the story must have gotten lodged in Helen's subconscious and that she plagiarized it unintentionally. Unfortunately, a teacher at the school managed to convince him that Helen and Miss Sullivan were lying. Helen had to go before a "court" devised by the institute and defend herself. Helen was distressed by that and also by losing Mr. Agnagnos's respect. The entire episode, which made her look sneaky, fraudulent, and dishonest, cast a cloud over her childhood.
The incident made Helen stronger, she said, because she became much more careful to make sure her writing was her own, but more importantly, she emerged with "a mind made clearer by trial and with a truer knowledge of life."