Do some honest analysis of the situation. Sit down and think back to where it all went wrong. Was it your fault? Was it something you said or did or didn't say or do although you should have? It is important to think about your inaction at certain points such as all those times you failed to make a call to see how your friend was, or when you missed appointments, or failed to show up at important events. Think carefully and do not make irrelevant decisions.
Act now. The moment you're aware that the two of you start growing apart, do something about it. Waiting will only bring about an inevitable permanency of the loss of your friendship. Be prepared to be the one who makes the initial steps; your friend may be laboring under the belief that it's something you've done to harm the friendship, so it's not worth waiting around for them to solve things.