It seems I am losing all my friends

I am really upset because it seems my closest friend is making new friends and never wanting to hangout with me anymore. I graduated last year and she is still in highschool so we don’t see each other at school any more, but this whole summer we hung out ALL THE TIME. She has a new bf and idk how I feel about it. I want her to be happy, but I don’t understand why she never makes plans with me and when I try to make plans with her she’s always “busy” yet I see she is hanging out with other people. I probably haven’t seen her in about a month or so when we used to hangout everyday. I get that it’s school season now and all but even when we both were busy we would always make time to hangout. I just don’t understand what has changed. She was my closest friend and I guess maybe I shouldn’t be so close with only one person because that makes me more vulnerable to getting hurt.