Okay, this is what I believe: Well, in your dreams 你 should see your mind as a hotel room. During the 日 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 下一个 day, the lobby is clean again.
A nightmare is basically just a dream, but then one which 你 don't like. So I think you're keeping a secret about something and you're doubting if 你 should tell anyone? (just guessing here) The part of "make your life a living hell" is just the thought of 你 about thinking "what if I tell this".
