I'm genuinely curious about this, since you took the time to recount it for us (readers).

Did you and she simply drift apart and stop communicating? Did you each relocate, geographically, and simply never remained in touch due to your new respective daily lives?

Since you sound remorseful that you didn't make more of an effort to maintain ties with her, have you ever considered searching for her on social media or through online Yellow Pages -- if only to write to her and apologize for (what you believe was) your culpability in the lack of friendship that developed?