What were the fears in Helen's mind when she started writing The Story of My Life?

Helen was nervous about writing a biography because she didn’t want to look back on the events of his childhood too closely.
We often have fond memories of our childhood that may or may not be accurate.  Helen’s fear when writing her autobiography was that she would look too closely at the events of her youth, and things would not be as she remembered them.  Sometimes we remember with rose-colored glasses.

It is with a kind of fear that I begin to write the history of my life. I have, as it were, a superstitious hesitation in lifting the veil that clings about my childhood like a golden mist. (Ch. 1)

This superstition was that the magic of childhood would be lost by examining the memories as an adult. You may realize that things were not perfect, or not as good as they seemed.  Some things may not have happened as you remembered.
Helen also found that she did not have as many early memories as she would like, except for big events, because at the time she was writing her memories of more recent events were stronger than those of early events.

Besides, many of the joys and sorrows of childhood have lost their poignancy; and many incidents of vital importance in my early education have been forgotten in the excitement of great discoveries. (Ch. 1)

Despite all this, Helen Keller still wrote the story of her life.  She believed that people would be interested in hearing about how she overcame the challenges of being blind and deaf.  Her memories of learning how to communicate and cope with her limitations were part of her story, alongside the memories of growing up that everyone has regardless of whether they can see or hear.