Okay, this is what I believe: Well, in your dreams 당신 should see your mind as a hotel room. During the 일 your mind gets new experiences etc., those are guests. At night, while you're asleep, your mind puts those guests in their own room so the 다음 day, the lobby is clean again.
A nightmare is basically just a dream, but then one which 당신 don't like. So I think you're keeping a secret about something and you're doubting if 당신 should tell anyone? (just guessing here) The part of "make your life a living hell" is just the thought of 당신 about thinking "what if I tell this".
