What I've been learning lately:
"People come, people go - they'll drift in and out of your life, almost like characters in a favorite book. When you finally close the cover, the characters have told their story and you start up again with another book, completely with new charcters and adventures. Then you find yourself focusing on the new ones, not the ones from the past."
Sometimes, things don't work out the way you anticipated. Things change, people change. People come, people go. You may miss them, you may not. But, either way, their part of the story is over. Such is life! So, let them go. Close the book. And, accept it. There's new people to meet and new adventures to be made. So there's no time to dwell on "what was" or "could have been."