Please provide a brief summary of Chapter 14 of The Story of My Life by Helen Keller.

Little Helen Keller was very upset because she was accused of plagiarism, but she did not intentionally copy anything.  It was all about a story she wrote called “The Frost King” that turned out to not really be original.  She was just repeating a story she had heard, but didn’t realize it at the time.
As Helen’s teacher Anne Sullivan described “the beauties of the late foliage,” she wrote them down on her braille slate.

I thought then that I was "making up a story," as children say, and I eagerly sat down to write it before the ideas should slip from me. My thoughts flowed easily; I felt a sense of joy in the composition. (Ch. 14)

As an adult looking back, Helen Keller realized that “if words and images came to me without effort, it is a pretty sure sign that they are not the offspring of my own mind,” but at the time she was a child and she was just enjoying writing a story.  No such thought occurred to her.  The adults didn’t realize at first that the words weren’t her own either.
Little Helen gave the story to Mr. Anagnos as a present, and he published it in one of the Perkins Institution reports.  That was when they found out that the story was a lot like another story called "The Frost Fairies" by Miss Margaret T. Canby from a book called Birdie and His Friends.  They realized someone had read it to Helen when she was little, even if she did not remember.
She didn’t remember the book, but there could be no question about the source of her words.  Even though Mr. Anagnos forgave her, the unintentional plagiarism had a lasting effect on Helen Keller.  She refused to write fiction. 
The author of the book Helen unintentionally plagiarized told her that someday she would write her own great story.

But this kind prophecy has never been fulfilled. I have never played with words again for the mere pleasure of the game. Indeed, I have ever since been tortured by the fear that what I write is not my own. (Ch. 14)

Clearly, Helen Keller felt that this one incident tarnished her reputation so that anything she ever published would be put into question from then on.  It was also important to her that she include the story in her autobiography.  She wanted the facts straight.