This is for lore :
Demons came to Earth to posses the bodies of humans because they needed new bodies and and that caused the Demon Apocalypse. If you got possessed you could try to heal yourself but deep down you know it's impossible and you tell yourself that for reasure yourself.

If you want to create a character tell yourself that EITHER he is possessed or not. The possessed can be EITHER "himself" or completly possessed. If he is "himself" he tries to control himself because the demon