You start walking, looking back often, the key held to your heart, next to the longing to go back to what you knew and loved.
What's back behind that door shifts and changes, going on without you...as you go on in your live, outwardly moving on, inwardly holding that key, that longing, next to your heart.
Then, one day, a day you knew was coming, a day you braced yourself for happens, forcing you to open your hand to look at the key held there. Forcing you to face the reality that there is nothing that key to that door holds for you anymore and you MUST let it go...