1) Are you 100% certain that there is nothing else going on here. Many mental and physical issues can be caused by something not related to magick. It may be best to go to the doctors or phycologists to rule out anything more serious.
2) To cleans is to remove negativity, quartz placed on a person or in a room can help absorb negativity.
3) Sage, the burning of sage around a person can cleanse them, the burning of sage in an area can cleanse an area, but you have to be careful not to breathe too much in and not to burn it around pets and children.
4) If this entity (if it is an entity) is self made, then you can unmake it. I would try some meditation, centering etc to refocus yourself, if you do not fuel the creation of the entity, then it may well not exist anymore.