I don't know what has happened to me this school year. What happened? Why this year?

How do I know when it is time to let go of a friend? You tell me that I'll know when I know that they are back to how I met them--a mere stranger--but how do I know if they are a stranger once more? I can't shake the possibility that they might come back as someone more than a stranger, maybe someday, and I'm hoping soon, but another part of me quietly whispers, Nope, that's it. They're gone. C'est la vie.