Complex sentence: Self-help culture currently assumes that someone abused by narcissistic parenting as a child likely struggles with codependency in adulthood; if as an adult one is or has been in a relationship with a narcissist likely struggles with not knowing what constitutes a "normal" relationship.
Split into simpler sentences: Self-help culture currently assumes that someone abused by narcissistic parenting as a child likely struggles with codependency issues in adulthood. An adult who is or has been in a relationship with a narcissist likely struggles with not knowing what constitutes a "normal" relationship.