Some wisdom from stoicism and fantasy heroines got me through my worst moments. Marcus Aurelius writing, 'You have power over your mind – not outside events. Realize this, and you will find strength,' is a brutal but necessary reframe. It shifts focus from things you can't control to the one thing you can: your reaction.
On a different note, there's a line from 'Jane Eyre' where she asserts, 'I am no bird; and no net ensnares me.' That's not about being fearless, but about refusing to accept a cage built by others' expectations. It's a declaration of personhood, which is a solid foundation for confidence.
Finally, I return to a simple one from 'Dune': 'Fear is the mind-killer.' The whole litany is a tool. It doesn't say you won't feel fear; it gives you a script to walk through it. For an insecure person, having a script—whether from a book, a song, or a mantra—can be the first step to speaking in your own voice again. My old copy of 'Dune' is dog-eared at that page for a reason.