missing someone is a relative concept, associated with a sense of loss. so when i say i havent missed something in a long time, what i'm actually saying is that i havent experienced a sense of loss in a long time.
the sense of loss is relative to having something. how can you lose something when you havent had that something? therefore, when i say i havent missed something in a long time, what i could be saying is, i havent lost something in a long time and that is because i haven't had something to lose in a long time.
sometimes you're so used to living without something in your life that it no longer becomes a loss or a void. it becomes integral and a part of you so that you don't even realise you don't have it anymore. and you start to wonder whether that something ever belonged to you.
'i don't miss being happy. not because i am happy, but because i've forgotten what it was like to be happy. or maybe i never was, so i dont know what it is like to be happy. thus i cannot miss being happy when i never had happiness.'