The short answer is that you can’t. Not really, anyway. When someone is growing up with you, they’re playing a part in shaping the person you become. They’re putting a stamp on so many memories that you’ll likely never be able to let go of. For some, the friendships you hold as children into adulthood fizzle out and become something less magical than what you’d grown up with. In these cases, you might be ready to part ways, having become different adults than the similar children you once were. But you’ll never be completely rid of them.
It may take weeks, months, even years, but they’ll catch up with you. Suddenly you’ll see them in the TV shows you shared as kids. You’ll remember how you spent that one Halloween, when you each told your respective parents that you’d be staying at the other’s house. And then you find that old Etnies shoe box that once held the skateboarding shoes you’d both delved in to together. Now it holds every note, birthday card, and photobooth strip of your friendship.
And suddenly you don’t care so much about that big blowout from those months or years ago. A light bulb has clicked on and you see the time you wasted apart. You see that the harsh words exchanged were likely mostly fueled by anger rather than real feelings of contempt.
So you know, at that point, that you can’t completely say goodbye to them, not for real and not for good. Your best childhood friend is a real part of you and nothing really compares to the feeling of sharing a secret bottle of liquor with the same person you shared your first practiced kiss with.