One has to be Joni Mitchell's Big Yellow Taxi, and particularly this line: Don't it always seem to go, that you don't know what you got til after it's gone They paved paradise and put up a parking lot
That peculiar feeling of loss that you had something special, and just completely wasted your opportunity, an opportunity that will never offer itself again. So sad and bittersweet. Live in the past all you want, but that chance is lost forever.
