Being "Possessed" by one's own mental constructions is essentially mental illness, as through therapy and the like has an impact on these subconscious creations.
Otherwise I do very much agree with much of what was suggested above. Typically one cannot be possessed or otherwise inhabited if one has a strong sense of self. A possessing spirit must then find a weakness and exploit it, 'causing panic, confusion, and similar emotions. Often those who are possessed slip in and out of such a state, in which they also desire to sleep as their conscious mind is encouraged to "shut down."
Between these moments of panic and confusion are often gaps of time that the individual simply does not remember. During these times, they almost appear to have seizures on occasions. In others, their whole demeanor changes, presenting an entirely different body language, vocal pattern, etc. Within this you can also sense a change in energy that, frankly, is quite unnerving when combined with the rest.
To my experience mental illness can exhibit many of these same symptoms, but the person still seems like a version of themselves that is simply acting out. Much like every kid in the chatter crying about being possessed.