For example:
I try to avoid walking through the alcohol aisle when I buy something at the grocery store, even if I have to walk half of the store to avoid it. Although I would not buy alcohol nor have I ever have or will use it, I am afraid of catching a free floating spirit of alcohol.
This sounds like you may have Obsessive Compulsive disorder. Your obsession is the irrational fear of being possessed by demons, your compulsion (which is an attempt to make the obsession go away) is to avoid anything that you think may cause you to be possessed, no matter how crazy it may seem (like the not going out at night one).
I have Obsessive Compulsive disorder too. One of the main problems I had with it was I had to say the exact same prayer every single day and night "perfectly". I was convinced if I didn't say my prayer "just right" God would not answer it (see, irrational), so I had to keep saying the same prayer over and over, which usually lasted for a few hours. Now I just say the Lord's Prayer, it's short, simple and to the point and it's easier to say without feeling I said it "wrongly". Another thing I do is having to check the front door is locked over and over, which is annoying and also embarassing. So far I've been able to decrease my Obsessive Compulsions by will power, but it usually manifest's itself in other ways, for example, instead of repeating my prayers, I have to say "pardon me, there is only One Lord" (I know, it sounds completely stupid) if I make a noise, other than me talking to God.
Obsessive Compulsive Disorder is not caused by demons or anything supernatural, it's a mental illness, believed to be caused by lack of seratonin in the brain. Seratonin is a chemical messenger, which transmitts signals from the outer to the inner regions of the brain. What normally happens is, for example, you're going out, so you open the front door, then subconsciously think "I don't want anyone to enter my house and steal anything, so I better lock the door" - you feel anxious about someone breaking in), then you lock the door and everything is fine, you go about your business. But with OCD sufferers, what happens is, they feel anxious about someone breaking into their home, so they lock the door, but the "anxiety" signal, which normally passes to the inner region of the brain and goes away, "bounces" back and you still feel anxiety, so you check the door, and the signal gets stuck in a loop, so you keep checking and checking, thinking "what if I checked it wrongly, what if I broke the handle when I checked that last time, what if..., what if..., what if...."
You should not feel stupid for doing things, which may seem crazy to you or other people, they don't know how you feel. If anyone tells you that you are stupid or whatever, ignore them.
Of course, I could be wrong and you may not have OCD, but another clue you may have it is the fact you posted this looking for
reassurance that demons cannot possess you, which is another symptom of OCD. OCD sometimes comes coupled with another mental illness, such as depression.
This may help:
http://ocdcommunity.ifrance.com/ocdcommunity/
well said 
