What is the cause of Ellie's inability to fully love and trust her mother, Marion?

Ellie and Marion talk and laugh easily with one anther, but Ellie will never get out of her mind how her mother left them. When Ellie was a little girl, her mother left her and her siblings for three weeks. Marion left temporarily but suddenly again when Ellie was eight. Ellie fears that her mother will leave again, therefore she has serious trust issues.