Why your character keeps believing the lie
Your protagonist keeps their false belief because it is paying them something real, and the payout should be good enough that someone in the story admires it.
Why fixing your character's worldview isn't enough
Under your protagonist's rule for how to live sits a low opinion of themselves, and an ending that fixes only the rule lands early.
Fix what your protagonist believes about the world but not about themselves, and your story ends before they're done changing.
Your protagonist lives by a false belief that reads as a rule about the world, like "trust is dangerous" or "control keeps me safe." That belief is what StoryDNA calls The Lie.
Under the rule, your protagonist holds a low opinion of themselves. Not "people can't be trusted" but "I don't deserve real connection." The rule tells your protagonist how to act. The low opinion tells them what they are.
Your story can fix one without fixing the other. Your protagonist can accept that trustworthy people exist and still believe they're too damaged to deserve them. The low opinion is usually the last to fall.
People can stop believing something harsh about the world and still believe something harsh about themselves.
In Jaws, Brody's rule is to leave the shark to the experts and the bigger boats. Under it sits a low opinion of himself: that he's a coward who'll fail when it counts.
So write two sentences, not one: the rule your protagonist lives by, and what they believe they are underneath it. Then check that your ending breaks both.
When your character's best quality looks like a danger
The Lie makes your protagonist's best quality look like a danger, so the change doesn't add a strength, it points the one they already had somewhere new.