Helen Keller begins her story with these words because she harbors doubts about her ability to tell her story accurately.
Helen confesses that, from where she stands, "fact and fancy look alike across the years that link the past with the present." It is quite often the case that the child's impression of events differs from that of the adult's. Helen admits that "the woman paints the child's experiences in her own fantasy." She is perhaps alluding to the fact that it is difficult to accurately represent events from long ago. Either the "joys and sorrows of childhood have lost their poignancy" to the adult mind or the "many incidents of vital importance in...early education have been forgotten in the excitement of great discoveries."
What Helen means is that, at present, she is as much afraid of idealizing her past as she is of being inaccurate in her narrative as she tells her story. Last, but not least, she doesn't want her story to bore her readers, so she says that she will "try to present in a series of sketches only the episodes that seem...the most interesting and important."