From what you tell us what she told you, it seems like she feels she isn't gaining as much from your friendship as you are, in a way.

Sounds like in a way she was your friend out of charity or convenience and now decided it doesn't benefit her or you any longer.

Note that this is interpreting what you told us what you told her based on the few things you describe here. This analysis is extrapolating a lot.
But she does say you have insecurities according to her, and you describe being anxious and crying. Not saying this isn't normal when a 8 year long friend ends a friendship.

I don't think you can say she offered no explanation, because she did. Just not a very detailed one or one you can learn from.
But she did tell you that she did enjoy her time, but maybe that aspects of a deeper friendship in her mind, where unequal. It seems she didn't feel you made her grow as a person but you did depend on her?
It doesn't sound like she feels comfortable with telling you more about why she wants this friendship to end. She must know that words of criticism from a (former) friend hurt the most.

If she doesn't want to talk to explain things further, I doubt she will be willing to do it at her job when you run into her.