Dear Best Friend,

I am writing this letter to tell you a lot of things that I wouldn't be able to say out loud, which is really saying something, as you know how vocal I am. But there are some things, like you say, that are best left as written words, never actually spoken.
Everyone knows that friendship is a bond that stays with a person for a lifetime, either as a relationship or as an unbroken strand of multicolour memories. But I can see the reflection of our impending separation in your face these days. Every time we laugh together, or say things at the same time, or have a deep meaningful conversation about things that we normally prefer not to share, we both look away, and I guess you're wondering the same thing: what we would both do when that goodbye is done and Facebook will be all that keeps us connected.
I know I'll never be able to find a companion like you. I can't even hope to. The very idea is laughable. There will be no one I will text at midnight on their birthday, no one I wouldn't hesitate to call at any time simply because I'm bored, and no one who would dissect my neutral expression and know there's something wrong. I know it, because I've already had the miraculous, perfect friendship that people crave all their lives. Nothing can be better...and nothing can be worse.
But I want to let you know that I'm not letting you go. You will always be with me, a pseudo-presence at the back of my head. I will always think of you, guess what you would say when looking at a particularly complicated problem, imagine the smile you give me in reaction every time I write something and give it to you to read, and your frown when I'm about to do something I already know is stupid.
And I will always love you for who you are, because like you say, that is the purpose of friendship: having people love you without wanting to change you.
Let'd promise each other to stay in touch like everyone does, and actually keep the promise. Let's say that we'll recognise each other if we happen to cross paths years later and not just walk by when it actually happens.
Let's remember each other, and not as a forgotten thing of the past.

I'll miss you.

Love,
Your Best Friend