What is the biblical definition of being demon-possessed?
How would you differentiate someone who is demon-possessed from someone who is simply struggling with sin?
A truly possessed person has no control over their mental abilities to choose their own situation any longer. This is why true possession is so rare.
There is no actual biblical definition, but we see in the writings of the early church fathers that more definition is given. The 'Gazarene' man Jesus freed was truly possessed, he no longer had any personal control over his circumstances. Jesus literally had to drive the demons out. But the man in the temple Jesus freed was not possessed, but 'oppressed' as a demon was 'atttached'. Jesus told him to "leave that man". The man, himself, was still able to choose to attend temple. A truly possessed man would refuse to go near anyplace or anything devoted to God.
A person who has 'attachments', or is being dogged by demonic entities, is one who is constantly harassed by them, but still has control over what they choose to do and think. Those people may encounter paranormal phenomena, noises, interference at home or away with their thoughts or a constant mental assault on their senses or thinking process. They 'know' that the thoughts projected at them are not really theirs, but since the thoughts are so negative and aggressive, it's very difficult to block them out. They're usually thoughts that are simply 'off the wall' for a normal person to think, and most people dogged by this feel like they're losing their minds. At this point it's really wise to seek out a Christian psychiatrist or psychologist that actually works with people under oppression of demonic influence, as many of the symptoms can resemble mental illness, but be something else entirely. Ask your pastor or priest for a referral for a proper diagnosis. Nearly every Catholic Diocese has at least one referral they can give you.
Struggling with sin is a common thing for all conscientious individuals. That struggle is being either tempted often or constantly to say or do things that we know are 'not right' according to our own conscience, or feeling drawn to commit an act that we know is morally wrong and feeling a deep guilt or conviction within our own conscience that we even have to struggle not to do it.
If this is your state, pray for the strength, wisdom and knowledge to not fall into the temptation and take steps to limit your desires by avoiding the situation where you feel most tempted. If you can, find a therapist, counselor or prayer group that will help you understand why it is you are struggling so hard. It may be that your thoughts are not completely your own but projected. In that case, pray the prayer of renunciation and deliverance prayers daily and ask for deliverance prayers from your church as well.I would refer you to this link. It's been a great help to many of my clients.It's from the Padre Pio center for Deliverance Counselinghttp://saintpiocenter.org/materials/Personal.pdf
Understanding why you have these inclinations and how you can help yourself gives you the ability to focus your efforts in a rational and substantial way.