When discerning whether someone has a demonic problem or not, I typically ask what "messages" they are hearing in their heads. Over the years, I have heard the demonically possessed and oppressed consistently relate six basic messages. Satan whispers (or yells!) these in people's heads without respite:
You are a terrible person.
There is no hope for you.
God doesn't care about you.
You're mine. I will never leave.
You are going to hell.
You should kill yourself.
There is a little bit of this mental negativism in all of us, we who are tainted by Original Sin. But when Satan is doing it directly, the message is loud, consistent, and unrelenting. I personally believe that more than a few people take their own lives after being worn down with years of this mental battle.
What to do? I recommend people confront this on both the natural and supernatural planes....
On the natural level, Satan gets a foothold in people's psyches through their human weaknesses and sins. In this case, the more damaged our psyches, the stronger this negative self-talk in our heads. Satan will exploit this weakness.
So, we should engage normal human remedies for such mental negativity. For example, a variety of cognitive-behavioral interventions can be a help. There are a plethora of these on-line. Counseling by licensed professional, who is supportive of the faith, might bring some healing to the roots. For more serious cases, medications for obsessive-compulsive thinking may be indicated.
Ultimately the final antidote to Satan's message is the Good News of Jesus. This supernatural battle can only be finally resolved on the supernatural plane. Once we know deeply in our hearts that God loves us personally, and that we are saved by the blood of the Lamb, then our minds can fully be at peace.
There is no other final remedy to Satan's bad news than the Good News of Jesus.