My DD9 often expresses the same thing. She's a perfectionist who often pays too much attention to the negative in everything, and that means she turns it on herself as well. I can ask her to name one thing she likes about herself, and she won't answer. It's really sad, because she has so much going for her, and yet I can see at times where her lack of self-assurance becomes self-defeating.
Other times she's entirely self-confident, so it seems to be a mood thing.
Lately we've been working on her paying more attention to the good in other people, and we've seen a change in her behavior through that. Hopefully she can turn that change towards her introspection as well.
Yes-- and getting them outside of their own heads a bit can help, as well. Being in an echo chamber full time isn't a good space for some people.
I try not to make too much of this kind of angst-ridden statement from my DD. (Inside, I'm thinking;
What? Are you Holden Caulfield or something?? Sheesh! But outwardly I try to be more patient with it.)
Journaling
can be good for children who have feelings that are private but need to get "out." On the other hand, for already-introspective kids, too much navel-gazing can go bad places. My DD seems to be one of those-- though she does make statements like this occasionally when her mood is all wrong and everything about the world (and her in it) feels awkward and mismatched.