possessed - by a familiar spirit

I have never thought much about the whole idea that one can become possessed by evil spirits and the like - it's never been something that particularly interested me. Certainly not in the traditional way it's portrayed in scary movies and other paranormal media. I have thought about it though, but more about possession in relation to emotional states, attachments.. I know one thing, being possessed with living people is a lot more common than being possessed by dead people - and likely to occur at least once everybody's life.
but dead people??? I really don't know.
However, today i have been forced to think about this a little... in a bit of an obscure way.
What would it mean to be evilly possessed by your very own mortal soul?
I know it's boring listening to people tell you about dreams they had, but i'm been really thrown by this one.
I woke twice this morning, the second time in a mild sweat with a rapid heartbeat and feeling of impending doom. I was in the throws of a nightmare which although had occurred within the previous 20 minutes (since my first waking) felt like it had lasted three or four days. I dreamed of this house. I was in it and there were visitors and we were all being held hostage by this black black spirit of a girl. The omnipresence of this dark force was overwhelming and no movement in the house was permitted, no thoughts permitted, trying to do anything was like wading through very think molasses or tar. From an old man came the advise that so long as we tried to stay human and separate from this entity, we would never be free. The only way to gain freedom was to join with the negative energy and let it live through you.
once i had accepted this I faced the terror of meeting with the girls spirit. she came towards me and as she got closer I realised the deathly girl was none other than myself. I was utterly terrified and perplexed but it was too late to escape as she clutched my hair, my face and arms with her chill and fused with my body with a big jolt. This is where i woke, panicked, believing that i was possessed by the most evil blackness.... my panic was then enhanced when i remembered that this evil blackness was me!!

not a very nice dream to be interpreting

I have not yet been able to find anything online about the idea that your own spirit could be haunting you. To me it doesn't make much sense since you are always possessed by your own spirit, other-wise you would be dead (dead like the spirit i was dreaming of?)
it could go round and round and round