focus on your breathing. Its really hard to clear the mind of its all thoughts we process a lot of shit at once, so the best way to calm down is to try and focus on just breathing. Taking really deep breaths and then letting out slow. Try and get in to a rythm don't count it out or anything just try and keep and your breathing rythmic. OF course turning off the lights and or closing your eyes always as making sure your comfortable is important but i'm sure you've tried that allready.