Her ability to fend off the inner personalities weakened over time. As a child it was strong, then grew less and less as she faced more and more pressure as Regent.
In that knowledge lay recognition of a terrible reality—Abomination.
The totality of that knowledge weakened her. The pre-born did not
escape. Still she’d fought against the more terrifying of her
ancestors, winning for a time a Pyrrhic victory which had lasted
through childhood. She’d known a private personality, but it had no
immunity against casual intrusions from those who lived their
reflected lives through her.
Thus will I be one day, she thought. This thought chilled her. To walk
and dissemble through the life of a child from her own loins,
intruding, grasping at consciousness to add a quantum of experience.
Children of Dune
Alia made things worse by regularly consuming increasing levels of spice to give her better access to her 'inner voices' and eventually life-threateningly large amounts so that she could read the future.
Pressures of responsibility had driven the old fears away and she had
been wide open to the inner lives, demanding their advice, plunging
into spice trance in search of guiding visions.
Alia made things worse by consulting with her inner lives, rather than ignoring them or actively resisting them.
The crisis came on a day like many others in the spring month of Laab,
a clear morning at Muad’Dib’s Keep with a cold wind blowing down from
the pole. Alia still wore the yellow for mourning, the color of the
sterile sun. More and more these past few weeks she’d been denying the
inner voice of her mother, who tended to sneer at preparation for the
coming Holy Days to be centered on the Temple.
The inner-awareness of Jessica faded, faded … sinking away at last
with a faceless demand that Alia would be better occupied working on
the Atreides Law. New lives began to clamor for their moment of
Alia made things worse by consulting with the Baron and giving him increased notice, eventually resulting in his capture of her personality.
Alia felt that she had opened a bottomless pit, and
faces arose out of it like a swarm of locusts, until she came at last
to focus on one who was like a beast: the old Baron Harkonnen. In
terrified outrage she had screamed out against all of that inner
clamor, winning a temporary silence.