Amethyst from Steven Universe.
Now, this is an interesting pick because, when I first started watching the series, I liked Amethyst because she was the person I wanted to be, rather than the person I actually was. She was chill and didn't give any fucks about what anyone thought of her. And I thought that Pearl was a lot closer to my actual self -- being high-strung, stressing about things I can't control, and being all kinds of insecure.
However, as the series went on, we saw that Amethyst has plenty of insecurities, too, and that her cool persona is often a facade that's meant to hide all of that. Not only that, but the specific things that Amethyst is insecure about are all things that I can really relate to -- being "defective", and not as strong as other gems, and feeling like even her best efforts won't ever be good enough because she just wasn't built right, and how she sometimes feels so useless as a result...
And so, not only can I relate to her feelings of inadequacy -- I also feel like her way of dealing with it is somewhat similar. Much of the time, she covers it up and puts up this cool, gives-no-fucks facade (which is often how I act IRL...), until something triggers all of that emotional baggage to come to the surface, and then she beats herself down for not being good enough...