Clearly, if someone is not in your life anymore, you miss them - but how can you miss someone who still is?
That made me think about "what" we're missing more than "who" we're missing.
You'll miss the special things you did for each other - and no one else.
Probobly think about how you felt when you were with them - how that feeling was unknown before and is again, now that you're alone.
Their smile, their voice, their touch, the way they looked at you with their eyes full of desire - wanting to take all of you.
I'ts appears to me that these are the same things you'd miss about a past relationship as much as from a current one.