It's self-contradictory only in the same way as "I believe a lot of false things" is. (Maybe a closer analogy: "I make a lot of mistakes.".) In other words, it make a general claim that conflicts with various (unspecified) particular beliefs one has from time to time.

I am generally underconfident. That is: if I look at how sure I am about things (measured by how I feel, what I say, what in some cases how willing I am to take risks based on those opinions), with hindsight it turns out that my confidence is generally too low. In some sense, recognizing this should automatically increase my confidence levels until they stop being too low -- but in practice my brain doesn't work that way. (I repeat: in some sense it should, and that's the only sense in which saying "I am generally underconfident" is self-contradictory.)

I make a lot of mistakes. That is: if I look at the various things I have from time to time believed to be true, with hindsight it turns out that quite often those beliefs are incorrect. It seems likely that I have a bunch of incorrect current beliefs, but of course I don't know which ones they are.

(Perhaps I've introduced a new inconsistency by saying both "I am generally underconfident" and "I make a lot of mistakes". As it happens, on the whole I think I haven't; in any case that's a red herring.)