Have you ever felt like you've lost someone, even though they're still sitting right there, looking at you and drawing breath?
Sometimes in life we create an idea of someone in our lives and its not who they really are. Its who we think they are. Or maybe who we know, deep inside our hearts, that they're not, but convince ourselves they can be, the person we want them to be. Its usually with someone important to us. We convince ourselves they are or can be something or someone that they simply aren't, as a survival mechanism usually, so that we can excuse/ignore/accept that they aren't giving us the things that we need. So we can pretend that if we just hold on a little longer, they'll figure it out and be the (insert relationship here-- friend/sister/brother/lover/parent) we wish they were.
But something funny happens. Heavenly Father gives us the tools to find elsewhere whatever they aren't giving us, even if they're the ones we should be getting those things (affection, dependability, validation, honesty, support, love, etc) from. He provides His own love and also very frequently puts other people in our lives to make up for the deficiencies. And we learn to stand on our own a bit more. And we learn to trust and rely upon the right people, little by little. Finally, one day, we realize that we can get by just fine, even happily and contentedly, without whatever we thought we should be getting from that person. And then the person we thought they were dies: that idea dies, and we finally see them just as they are, because we don't need to pretend anymore. We still hope. We may still try to reach out. But we make our peace with the fact that they may never get it. They may never be the person we want them to be, or think they ought to be. Perhaps they aren't even capable of being that person. And that's OK, because you're not relying on them anymore, and you know there are other people in your life who can provide you with the things they can't.
And yet, just like when we in reality lose a loved one, the longing doesn't ever entirely go away.