It's surprising how much losing someone you thought of as a good friend hurts. I'm going through that at the moment and it feels just like breaking up with a partner except without someone to talk to about it! I think part of why it's happening for me is because my life is changing and theirs isn't, I think they may be jealous and sub-consciously trying to sabotage me. It's painful but I have to believe that the space that losing my best friend has created will be filled by something beautiful.
I liked your quote, if your current friends can't say that then new ones will. x