Letting go...

I have this problem. You can ask my husband. I have a hard time letting go of things... whether it's letting go of the point (his favorite thing: I just can't stop explaining why he was "wrong" during an argument), letting go of control, or letting go of people.
That saying about people coming in and out of your life? The one everyone emailed around a few years back? I think it's a lovely thought, and apply it beautifully to people who have left in disagreeable circumstances or who were, from the very beginning, known to be a transient person in my life. I always think, "Hm. That was a lesson learned." Or in the case of those people I knew would only be in my life for the weekend or party or whatever circumstance, "That was fun!"
It's the people who call themselves "friend" I just can't let go. Not in a stalkerish, crazy, hunt them down kind of way. Just in a I feel so sad because he/she/they no longer cherish our friendship as a here and now kind of thing but have moved past it and consider it a fond memory. (Or so I hope!) Nothing went wrong. There were no harsh words, hurt feelings, betrayals or other dramas. They just stopped caring to know me in the present. They moved on with their lives while I hold them close and think we are still friends and wonder, hurt, why they no longer want to chat or care what's going on in my life. Rather than being the person I talk to on a daily basis, the person I know everything about, these lost loves are Christmas card recipients, FB and MySpace "friends"... We say we should "totally get together when you're in town" but never do.
Is it some psychological issue that makes me cling to that memory? That makes me wish we were as close as we once were? That just can't let go and move on? I mean, it's not as if I don't have any other friends. And we don't live close to one another... So why am I so determined to never let them get too far away? Why am I so hurt by their ambivilance? Isn't it just part of life that we move on, make new friends, remember old friends fondly, etc etc?
I'm tired of being sad about people who've chosen to disregard a friendship. It's not difficult, especially in the age of FB and email, to keep a friendship alive, to update one another on our day to day goings on. If they wanted to, they would and could still be my friend in nearly everyway as before. Guess I'll just have to figure out how to let go and stop being sad about it.