You know if this had been someone I just casually hung out with on occasion then it wouldn't have bothered me that much but this was someone I grew up with.
I have that EXACT same problem right now. I know I'm always complaining about having not heard from my favorite person in a year now, but the problem is I like her. That's the problem, because there are other friends I don't hear from for extended periods of time as well, and it's no big deal, but when I don't hear from her for a long time, it is a big deal.
Of course, part of the problem too is that she really drops off the face of the earth for no rhyme or reason... others I at least know why I don't hear from them as much, whether they're swamped with work, college/university, life in general, etc, and some, like my best friend, at least do make an effort to try to contact me every once in a while, and they'll even say, "I know we don't talk very much anymore, I'm sorry, but I'm so buried over here...", which is fine... but with She-Who-Shall-Not-Be-Named, there's NEVER an explanation of why she goes into these prolonged hibernations, she never reaches out to me, I have to badger her, and then if I finally get a response, it's like a, "Oh yeah, hey, what's up?"
Then again, her social skills do seem somewhat askew, I think it has to do with her childhood... that, and she's so OCD about the boyfriend she lives with, that she's probably always clinging onto him, so... whatever.