Helen Keller's The Story of My Life begins with the sentence "It is with a kind of fear that I begin to write the history of my life." Elaborate.

Keller begins by articulating her fear of writing her life story, saying that autobiography is a "difficult" task. She goes on to observe that an adult looks at childhood through a different set of lenses than a child does. She fears that in writing her memoirs she won't be able to accurately convey a sense of what it was like for her to be a child. As she puts it:

The woman paints the child's experiences in her own fantasy.

She notes the child's "joys and sorrows have lost their poignancy," meaning that it is impossible to feel the emotions of her childhood as acutely as she did at the time. She also worries that she will have forgotten a good deal of what happened to her in childhood.
Because of the many ways an adult can distort events as she looks back on them, Keller says she will try to include only "sketches" of what stands out in her memory as most "interesting and important."
In terms of evaluating this opening statement, it is important that Keller is self-aware enough to know that her adult self will have a difficult time writing an accurate memoir. This raises the credibility of the autobiography, in my opinion, because it suggests that Keller will be careful to be as accurate as possible. It also indicates she will try as hard as possible not to impose her adult consciousness on the child's mind, all the while being upfront about the impossibility of completely recapturing the experience of her younger self.