which is which?

It's hardest at night. When the stillness sets in. At the end of a party, when everyone's left but the decorations are still hanging. When everyone's gone to bed, and I'm still awake. The past haunts me, the future daunts me. Yet, the present is the worst because there is nothing. There's nothing for me here.   I'm not saying it's not worth sticking around for. Of course whatever's out there is worth waiting for. I suppose a part of me is afraid to be happy. Even for just a moment. It used to be so easy. Now, even in the happiest moments, on the sunniest days, a voice in the back of my head tells me that this won't last. That the darkness will return, stronger than before. Ghosts of what should be, of what could be, tell me that the happiness isn't worth its price: the unbearable sadness.
What if.. sadness and happiness are twins? Completely identical from the outside, but radically different on the inside. Or perhaps it's not so much the nature of the inside that's different. What if the qualities of sadness were brought about by nurture rather than nature? Maybe, sadness is happiness under different circumstances.