Where am I?

  I look into the mirror,
  And I don't recognize,
  The person staring back at me,
  She looks so much older, 
  She looks so much paler,
  She looks so much weaker,
  Where am I?
  And who is this girl who has replaced me?
  I look in the mirror,
  And what do I see?
  A girl who looks so different from me,
  She's taller,
  She's paler,
  And so much weaker,
  That really can't be me,
  She looks like a zombie,
  And acts like one too,
  But where am I?
  Am I gone forever?
  Is she all I can be?