Why do the old crones encourage Annie to keep her past hidden?

At various points in the play, Annie is plagued by little voices in her head. In addition to a little boy's voice, she hears the insistent sniping of old crones (an old-fashioned way of saying "old ladies.") The crones crop up at regular intervals, undermining Annie's confidence in herself and her ability to teach Helen. The voices are echoes of Annie's past, when she attended a school for the blind.
Early in the play, Michael Anagnos, the director of the Perkins Institution, advises Annie to mention her own experience of blindness when she meets with Helen's parents. However, Annie wants to be judged by her abilities, not her disabilities. In any case, she already knows that her relative youth and inexperience will come as a shock to the Keller family, and she does not want to make a potentially bad situation worse.
The crones, as the voices of doubt, give Annie a message which is the exact opposite of that of Mr. Anagnos. They warn her not to divulge information about her background. Deep down inside, Annie is still a little unsure of herself, and she is not completely confident that she will be able to break free from her own history of disability and be able to assist Helen in achieving her educational goals.