You'll wrestle with the little things -- what to do with all the framed pictures, or how to acknowledge her birthday. You won't want to throw away the photos or ignore the birthday, but you also won't want to do what you've always done, acting as if everything is normal -- because it's not.
When something exciting or awful happens, you'll immediately want to tell her and hear her reaction to the situation. Then you'll remember that your friendship isn't like that anymore. Your heart will sink a little, and you'll have to tell someone else -- but someone else just won't get it, so it's not worth it.
You'll face new-best-friend jealousy. This occurs when she starts treating someone else the way she treated you, when she chooses to share special moments or days with another person, posting inside jokes that you no longer understand. Each time this is shoved in your face, it will hurt as much as the first time. That little pang of envy will never really subside.
The smallest things will spark memories of your 15-year friendship, and almost every single moment of your childhood involves her. Your friendship will be impossible to disregard because she shaped the person you are today.
Other people will ask, "So how is _____ doing these days?" because as far as they know you are attached at the hip. But now you won't know how to answer because, well, you don't even know her anymore, so how can you tell someone else how she is doing? You will likely end up explaining this to some extent, which will turn awkward quickly.
You won't know who to call your best friend. Sure, you have other close friends who you tell almost everything, but after having a best best friend like her, no other friendship will come close -- after all, not many other people have known you since you were six, have experienced the awkwardness that is middle school with you, have listened to you blubber after your first breakup. Those moments and the ensuing friendship cannot be duplicated.
You'll miss her more than you thought you could miss someone, the hurts-your-stomach-to-think-about-it type of missing her. You'll hurt more than any breakup with a boy would hurt you. You'll wish so badly you could have handled things differently so that it didn't lead to this, but you'll know you can't undo any of it now.
After the fallout, you two might get back to the point of being sort-of friends. But even this will feel forced and awkward -- something you never would have thought could be an issue with this person who knew every single awkward, inappropriate, mortifying aspect of your life. But you'll take what you can get, because a little piece of her and your former relationship is better than no piece at all.