if you try to sleep on, say, ottoman, do you feel better? I had the same problem: could not sleep horizontally. If I try to sleep on ottoman, when the back is slightly raised, it was possible. I read somewhere this happens because of blood pressure or blood supply to the brain. it increases when one is down completely (horizontally).
I was watching a movie, when was trying to sleep. It kind of takes you away from your thoughts. Just low the volume.