How do I close myself off from demon possession?

When I was little, I had an imaginary friend that I called Nightmare. I always saw him as a big black dog, and I could imagine him very vividly. He was always with me for a long time, and then he suddenly disappeared. He'd always served me as sort of a guardian figure in the absence of my father. Roughly a year ago, Nightmare showed up again. He looked different, but he felt the same. He spoke to my boyfriend through me sometimes, and we were all on very friendly terms. But after a while, I got kind of worried. Nightmare started taking over my body without my permission, and tainting the atmosphere in the house that my boyfriend and I share. Nightmare started trying to seduce my boyfriend, and darken our thoughts toward one another. He's been filling our heads with evil thoughts, and making me think and say dark things. We've been burning incense, praying, and laying olive oil on our walls, and it's helped lighten the atmosphere considerably. But Nightmare still manages to force his way into my body sometimes. He'll say thing like 'you can;t get rid of me' and the like. I need a way to make myself 'less open' spiritually. I've always been prone to attracting spirits and the like, but at this point I'm concerned for the safety of myself and my boyfriend as well. My memories are always very blurry when Nightmare takes over, and sometimes I black out and wake up somewhere else. Please help!