I had this very same experience years ago as a senior in high school. My best friend of 15 years (yes, we became friends at age 3) suddenly stopped talking to me...no reason, no explanation. We were like close sisters who did everything together and shared every feeling and secret that we had..with each other. She started hanging with others who were not like her and she became one of them.....It's been over 30 years and though I know I should have let this go decades ago, it still puzzles and hurts me. I have had many other friends over the years but I find that I have never wanted to get too close for fear of this pain happening again. I hope that you are able to let go. I know from experience that it doesn't help to let it control who you are.