I hope some of the advice here has offered some help to you. As psychosgirl said, demons are often a manifestation of negativity, but are not actual
beings. I had almost the exact same experience you described (though I only had this happen a couple times- once badly)...
I was under a great deal of stress/anxiety last year, and experienced something that could be considered a variation of sleep paralysis, almost
exactly as you described. I woke up to something pinning me down and hurting me and in the scariest, most sinister voice I ever heard said it would
never go away from me.
I talked to a psychologist about it, and she put my mind at ease, letting me know that I was basically having a lucid dream/ night terror induced by
my stress and anxiety, and also in going over old traumas and abusive experiences in my life.
The demon that wouldn't leave me, was not a being, but it felt very real. The voice was so sinister sounding because my body was partially in dream
state while some of my senses were awake (so it was slower, deeper, drawn out and amplified). This explains why I couldn't really move or scream and
the like. After I was awake, I kept feeling a sinister presence there, but I now know it was merely my anxiety and fear from the waking dream.
The entire next day, I was so on edge and convinced my apartment was haunted. Objects were literally flying around, things falling off shelves..I was
just so high strung that I was involuntarily making my environment mirror the chaos in my mind at the time. The negative mental/psychic energy was
strong enough to take on a telekenetic effect...Never underestimate the power of the mind.
I realized, that it was all mind over matter for me. That demons, are fear, guilt, things haunting the mind, not home, etc. The best way to get rid of
such beasts (in my experience) is to:
Talk to someone about it, and read/research this. It is more likely that it is medical than paranormal. (I see the majority of replies you have
gotten, suggest this as well)
Meditate/find inner peace and balance. Feel yourself protected and surrounded by goodness, which no evil can penetrate. Pray, if it helps. Make a
"safe place" for your thoughts. Think of the most beautiful comforting and serene scene possible and "go there" whenever the fear and negative
evil=negative energy. The best way to rid yourself of negative energy is with happiness and love/light/laughter.
There are a lot of threads regarding sleep disorders, namely sleep paralysis on this forum. I would do a little search and click any resource links
the posts may provide. I think you will find that there are no actual demons, but only the ones we create ourselves.
Best of luck in freeing yourself from this opression. You can do it