Are you seeing a counselor or therapist? It may not be demonic. It may possibly be something like DID or schizophrenia or both. I'm no doctor and can not diagnose you but, I have a personality in me who is very dark and seems demon-like to the rest of us in my mind. Knowing that we feel that way boosts his ego and he likes to think of himself that way too. Until you start seeing a doctor or therapist about this it is going to be hard to know what is really going on but, don't just assume that it is definitely demonic in nature. With your already existing problems it may very well be a mental problem that just needs to be observed by a professional therapist and/or doctor.