You can be possessed by repeated mortal sin, especially of a sɛҳuąƖ nature. But generally, it is not common. If you can say the creed aloud, kiss a crucifix, and sign yourself with holy water, you are not too far gone. That is the test I perform on myself whenever I have a sinful streak I have to deal with.
Remember, demons are like vicious little piranhas and leeches. There is no sense of fair play or justice. They will attack and violate innocence at any age and in any condition and instill the most diabolical of thoughts into any person who is careless enough to mull them about, regardless of their relation to the victim.
I have been very sinful in my life, EXTREMELY so...but i have been conscious about Satan, and I have deliberately WILLED to not allow him to posses my body. He cannot violate your will. Your will needs to actually allow him in for him to possess you fully.
However, I do believe the majority of Christians are demonized. I believe that the majority of Christians has at least one or two demons that are committed to ruining their life, and that they slip in as far as they can as often as they can, but not as often do they fully possess anyone.