Best friend's devotion is hard to understand

I realized today just how lucky I am to have the best friend that I do. We'll call her N. She's unbelievably kind and accepting. We've been friends for over 3 years now. She's stuck with me, back when I was in 7th grade struggling with my sexuality, to now, with my gender. She's completely accepting with everything about me, and stands up for me in front of my somewhat less accepting friends. She's kind, funny, supportive, and pretty much everything you could ask for in a best friend.

But why?

As a person who likely has depression and anxiety, it is so, so difficult to believe that she cares about me. What could I possibly have ever done to deserve such an amazing friend? I feel inferior to her, like there's no way I could ever be a good enough friend to her. She's just sort of perfect. Her only real weakness is her lack of self-confidence. How could I ever compare to that? I feel like I'm dragging her down with all my complicated issues. I feel like she deserves better than someone as troubled as me. And even in the worst of times, when I inadvertently began pushing her along with everyone else away, she came back harder. She tried to talk to me, she invited me to do things with her, for crying out loud she invited me on vacation with her! She's devoted. But what does she see in me that I don't see?

This thread isn't really asking a specific question; I guess I'm just asking for people who can relate or help building self-esteem in general with relationships.