You didn’t come to CAF seeking health advice. Let’s assume you have done/are doing everything to cover that aspect.
The sacraments are very powerful against the devil. Fr. Fortes, an exorcist, teaches that Confession is the best exorcism.
Sacramentals are powerful. Avail yourself to holy water, Saint Benedict medals, blessed salt, and whatever you can acquire.
Prayer is part of the answer. Use the rosary and find Catholic deliverance prayers.
Seek help from a priest.
Call your diocese, tell them you are having a problem and want to speak with an exorcist. If they don’t have one, they may have a deliverance ministry. They’ll put you in touch.
The “why” could be important, but not necessarily. Have you had any cult involvement? It must be rejected. Any unconfessed serious sin? Any habitual sin? Stop sinning. Any family members involved in occult? Any family who may have had similar problems? If “no”, then don’t be too concerned. The remedy will be the same.
I will pray for you.