How did this event have a lasting effect on Helen and her writing?

From the text:

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. For a long time, when I wrote a letter, even to my mother, I was seized with a sudden feeling of terror, and I would spell the sentences over and over, to make sure that I had not read them in a book.