Dear Roberta,
It is not likely that you can reassure your daughter any more than you probably already have. She clearly has some anxiety about being left alone, and this anxiety persists even though you are trying to be sensitive to her. Her condition is a variant of
separationPlacenta abruptio
Separation anxiety anxiety
disorderAdjustment disorder
Anorexia nervosa
Asperger syndrome
Autism
Autoimmune disorders
Bipolar disorder
Bipolar disorder
Bleeding disorders
Borderline personality disorder
Bulimia
Chronic motor tic disorder.
I think you should make an appointment for her (and you) with a child
mentalMental retardation
Mental status tests health professional. The clinician can achieve a diagnosis and, more importantly, guide you about how to proceed. It might help if your daughter participated in therapy for a short while; it's hard to say. At the very least, you can receive some guidance specific to your situation.