I am a Christian believer who may have?

Yes, you might have a mild bipolar disorder, in which case it's biochemical and has NOTHING to do with demons or possession or any other kind of supernatural thing. If you don't feel your psychiatrist is dealing with your problem sufficiently however, find another.

No, you are not "possessed." You have bipolar disorder. It's a medical condition.

You have bipolar disorder. You need to see a psychiatrist - not a priest.

Instead of asking us amateurs, seek professional help. NOW.

Belief in goodness should lead to practices that increase goodness reducing fear/anxiety and sin , depression is usually an escape from anxiety we cycle back and forth , women are said to be more prone to depression but this may be because they are so used to anxiety they do not recognise it until it becomes clinical , to be Jesus like is to increase goodness try mindfulness with a twist of goodness .

a lot of christians suffer from depression.

that's what comes from membership in a gloomy, depressing, mind-numbing death cult.

Seek professional counseling. Seeing a priest will sometimes just result in a child getting raped - why risk that?

Baptism for the Dead ( 1st Corinthians 15:29 ) will make the dead evil soul leave , or else he or she will be baptized with you so he or she can go to the Light if he or she has sufficiently repented of all of his or her imperfections ; other-wise the demon has to leave for ever .
Too many adults allow themselves to become possessed ( where a demon comes through your mouth and up into your brain and takes over about a quarter size area of your unused part of your brain . Scientist have even done scans of a possessed person's brain and saw that when the other personality takes over that spot gets filled with an unusual added supply of blood ) by someone " The Possessed Person " believes who can help them through this life ( but actually is just some evil dead relative or other person wanting to avoid the Light of God's fiery presence ( even as the Legion begged Jesus to cast them into the pigs , rather than into God's presence ) . But only my Church can perform this baptism . The Utah Mormon version is wrong . But you 1st have to make the required sincere covenant with the Father that you will always obey all of Jesus' sayings , read my answers , and then contact me @ P.O. Box 14 ., Independence , Mo. 64051 .

Someday you will die. Someday soon after that, no one will remember you were ever here