We grew apart during that time as she became an anti-war activist and I became more and more conservative. I supported the troops, our government and our religion. All of these things were an anathema to her. As the years passed we still kept in touch with an occasional phone call and always a birthday card. I had stopped sending her Christmas cards as she told me they were not appreciated she had become an atheist.
After the 2000 election she called me to tell that she could no longer be a friend of a person who she knew supported Bush. She could not be a friend to a person who had no morals and would support a man who had stolen the election. There is much I could have said to her at this point, but I didn't. I just said, "I am sorry that you feel that way and I am sorry that for all these years I really didn't know you." I gently laid the reciever down. I have never heard from her again.
This is essentially the story.
Sounds like you've been growing apart for years. Apparently you didn't have enough in common to balance out the differences. From your description, you lost her many years ago, it just finally became obvious and final. It also sounds like you were the tolerant one, and you didn't lose much.