Okay, this is just my understanding. Someone PLEASE correct me if I'm wrong. If you are a Christian, a demon cannot possess you unless you allow it - Make a conscious decision to say, Yes, I allow this spirit to use me.
HOWEVER, demons can and do influence Christians. They work through doubt, insecurity, and our own human nature. They are master manipulators. They can overtly suggest we do something, and use our reaction to do the opposite, which is what they want. They can also influence us through others. A lot of times, they manipulate our emotions to make us feel that lies are true (like feeling rejected when no rejection is meant).
Fighting them is work, but fairly straight-forward. Surround yourself with prayer, and with people who pray. Read the Word out loud. Contradict the lies - "No, they are NOT rejecting me. That is a lie, Satan, and I refuse to believe it."
After praying, and if you feel you need the support, get some Spirit-filled friends around, just tell the demon to leave. Jesus has given us this power, through His Spirit. Use the name of Jesus Christ, and don't ask, but COMMAND the spirit to leave you (or whoever) and your family, and not to come back. There's only two requirements, besides being a Christian - you have to believe that the power of Jesus will make it leave. It's your faith that empowers the prayer. And you have to say it out loud. While God can hear your thoughts, demons can't. Be loud, get angry. Stomp your feet. This spirit is trying to destroy you and your witness. Even Jesus got righteously angry.
If you feel the spirit's influence again, just say, "No, you can no longer bother me unless I allow it. And I refuse you. You are not welcome. Go away, in the name of Jesus." Then thank God for giving you this ability.
I had to do this with some deep-seated anger about my husband's disability. I've had to continue to declare my freedom from the anger, because they don't give up easily. But it does work.
I always thought it was silly, saying this stuff out loud. And I definitely felt silly at first. But when it hit me that this spirit was destroying my family and my relationship with my husband, and also keeping me further from Jesus than I wanted to be, I got mad. Then, it wasn't me and several other women praying, it was me and this spirit facing off.
Hope something in here helps.