Okay, some of this might just be me.
At any rate, people are generally understanding when a romantic relationship ends and when you are devastated, they get it.
There isn't anything for when a friendship ends. There's no process. There's no outlet. There's just an empty space where there used to be inside jokes.
It sucks when the person that you thought understood you better than anyone turns out not to understand you at all, when you've moved in such opposite directions that you can't even see each other anymore. It's craptastic when you look at the same shared history and you are not only not on the same page about what has happened, but you're not in the same book -- you're not even in the same library. And then one day you realize that it's just over. That's all. You fight with the idea for a while, but no matter how you try to bend it around, the reality is that it's done. You're done.
You try to be gracious but sometimes you're bitter. You try to be kind but sometimes you get angry. You say some things that you don't regret, but that you would not normally have said. And then you realize, maybe if you had said them sooner? You wouldn't be here. (Of course, you might have been here more quickly. There's no way to know now.)
There's no real way to explain to other people what has happened, either. With a romantic partner, when someone asks, you can say "We broke up." When a friendship ends, what do you say? "We don't talk anymore?" "We stopped being friends?" "Everything imploded?" If there's a proper term for that, I don't know what it is.
Much like a romantic relationship, though, you have to stop sifting through the rubble of memory and just let go. Put the pieces down. They are sharp and they keep cutting you. In order to heal, you have to set them aside and walk away.
You have to say goodbye. You have to figure out how and then you have to do it.
But you have to.