The mare who liberated you from the influence of Nightmare Moon and saved Equestria is a heroine. I... I'm just a librarian, the biggest claim to importance I have is being Princess Celestia's student. I find it difficult to reconcile the two.

[In short? She just plain doesn't have the self-esteem to see herself as a global heroine. Which... might be a good thing, really, I mean that's one heck of an ego.]