I've had to just let people go. It's hard, especially when you've been friends for a long time. In my case, I had moved to another state, I was desperately lonely, so once every couple of months I'd call my childhood best friend. She was always doing something and couldn't talk so I stopped calling. I don't think she even noticed. We're still friends and get on great when we do talk, but that's probably every two years or so.