What does Helen mean by the expression, " the shadows of the prison-house are on the rest? "

Helen Keller believed that the task of writing an autobiography was a difficult one as many factors would affect the narrative; the chief among them being her own fantasy, which overpowered the events of her past. As her past and present blended together in the narrative, so did the facts and fancies in them. She as an adult, wanted to paint the picture of her past through her mature perspective. However, as time went by and she made new discoveries in her adulthood, many events and incidents which were significant in her past lost their importance. Thus, Helen felt that she wouldn’t be able to depict her past accurately. She states that the memories of the initial years when she was blind are still under the shadow. By this, she meant that those memories are blurred and feels like they are enclosed in a prison and can't be recalled.