"You've always been a terrible friend."

Well, for one, there's no way this doesn't make you a hypocrite. Particularly the fact that it was sent anonymously, because that rules out the possibility that you're a friend trying to tell me something about myself that I need to know, that I could improve. Now you're just a person tearing me down, which I qualify as an act of a terrible friend. So...well, right back atcha.

Two, I already knew that. I say it all the time. Doesn't mean I'm not taking this to heart, and doesn't mean it doesn't hurt. Congratulations, you hurt me. That's what you wanted, yes?
It's harder when it's anonymous. Then I just imagine the faces of everyone I've hurt. And that's a lot of old scars to irritate.

But three, I didn't need this, but I will take it anyway, as a reminder.
I know I've been a terrible friend. I'm self-absorbed and live in a movie. Nothing is real and none of the characters matter. I'm the manic pixie dream girl that comes into your life just to leave it later. I don't mean for it, but in expecting it, I lessen the possibility of actually saving a relationship.

I get it. I'm a terrible friend. I've hurt many people along the way. I always go after the ones that don't want me, and leave the ones that love me behind. I know, and I'm endlessly sorry.

But I also know that I can't go back, and neither can you. I don't know who you are, but if we're currently not friends anymore, there's probably a reason for that. It's honestly sad that you haven't moved on (I've been there, I know) but I can tell you now that I have, and you'll feel better once you do.

I hope this, or your own message, has given you some sort of closure.
Some sort of satisfaction.
So that we might say that at least one of us benefited from it.