She can conclude either that the people she relies on for love are incompetent, malicious, or otherwise ill-suited to the task, and she is all alone in this scary world; or that she herself is to blame for, well, everything. As painful as the latter explanation is, it is far preferable to the other one, which paints a life-threatening picture for a young being with zero power or recourse. The first option is not an option at all. Better to believe "It's my fault; I'm bad," which lets you believe there's the chance that "if I work hard and be good, I will be lovable." Thus, even the debilitating belief in one's unworthiness, nearly universal among people with mental health diagnoses and addictions, begins as a coping mechanism.