It depends very much on your spiritual path. If you are Christian, I would suggest going to a priest of your own confession. If your faith is different, it depends of it. What I can tell is that, at least in normal circumstances, Demons can not read your mind. They can only guess from the outside, being old spirits, not bound to a body, they have a lot of experience.

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Perhaps somebody in your family has performed some black magic or harmed someone really bad. (I do not imply that yourself did something like that, you do not need to tell us that, of course.) Perhaps you or someone in your family granted unknown spirits power over themselves, being naive. I hear of people doing things like that sometimes. Regardless of those things, you need to find out, probably pretty fast.