Who your protagonist's ability is for after they change
The Truth doesn't give your protagonist a new ability; it changes the old one from protecting them to serving other people.
How to write a character flaw that becomes a strength
Your protagonist's biggest flaw usually isn't a random defect. It's a habit that protected them once and still works well enough to keep. StoryDNA calls it The Weakness. The Strength is the same trait, aimed at something better.
That's why the change is hard. The flaw keeps getting rewarded, it keeps proving itself right, and giving it up means giving up a defense that still works. When the climax is won by the very move the flaw used to block, your reader knows the change is real.
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Your protagonist's hardest sacrifice: a defense that works
What your protagonist gives up to earn their Strength is a defense that still works, which is why it's frightening and why it comes back as the last temptation.
Three ways a flaw keeps your protagonist's strength hidden
Your protagonist's Weakness keeps their Strength out of use in one of three ways, and the release has to undo that same way.