That "Demon" might actually be neither a demon nor merely a bit of negative thinking. It might be a semi-autonomous personality complex split off from the ego as a consequence of trauma. Integration of such complexes through psychotherapy with a clinical psychologist or treatment with another professional mental health-care giver, might really be the thing needed... Can't be buying this "driving out of Demons business", from what little I've heard of cultures which still indulge in such stuff, it's not always that pleasant. And I can't see how it can be anything other than harmful to an already disturbed mind.