Wow, it seems there's a lot of resemblance to obsessive thoughts I've dealt with/deal with here lately. Another individual posts about his fear of schizophrenia, and now you with the possession post.
I grew up with a parent who was fanatically religious (Catholic) as well as went to Catholic school. I don't tend to practice religion now, and my belief in a God tends to come and go..so I would say I am agnostic (someone who does not know for sure whether or not there is a higher power).
Anyway, given my upbringing, I too had fear of possession, and it manifested itself alongside my fears of going crazy...so talk about anxiety!! I agree with what you're saying, that, because some groups of people (religious or otherwise) belief in possession, it is hard to contest whether or not it is real... BUT:
Think about this: the bible (as an example) is a book written by many different people over years and years, who were trying to make sense out of things that did not make sense, such as skin disease, blindness, foaming at the mouth (rabies or epilepsy or even, mental illness?!!) and so the natural result of their belief system is that anything misunderstood or things that "look" or "appear" crazy are of the devil. After much scientific research, we know this is not true. If you were to do some research online, science contests possession in that, people who appear possessed are likely suffering from psychosis or physical symptoms of a disease... etc.. IN other words, it is highly unlikely.
So, the cognitive behavioral way of treatment to deal with this is to say to yourself, maybe I am possessed. Maybe I am going to be (insert thought or fear here)... and to deal with the unknown. I truly see OCD as a way of asserting control over our lives- people with OCD do not like to feel out of control because of the level of anxiety it produces. So we fight it off by checking with others if we are sane, or with others over their obsessions/anxieties and fears. But, if we were to let go of our close grip over control, we will likely feel a bit better.
I don't know whether this information helps you at all... I certainly hope so. I have had this fear for many years too, and am still here and functioning very well. :) No possessions to report yet. I will let you know if anything changes. ;)
All kidding aside, I hope you feel better soon. Without knowing your background (you'd mentioned English is a second language) or cultural history, it is harder to help you maybe, because I don't know all of the things that make you who you are- including religion, your social circle etc.