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.
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.
Your protagonist's flaw keeps their best ability unused in one of three ways. Your ending has to undo the same one.
That flaw is what StoryDNA calls The Weakness. It guards The Lie, the belief your protagonist built to survive The Trauma, the hurt they carry in from before page one. The ability it blocks is The Strength, and using it would threaten The Lie.
People keep a gift out of use in three ways. They stay away from anything that would call for it, they let something else handle the moment, or they sense it and bury it.
One: staying away. In Shrek, the ogre lives alone in his swamp, far from anyone who might ask him to let them in, so his warmth never gets tested.
Two: a substitute. Scout Finch in To Kill a Mockingbird settles things with her fists. Then at the jailhouse she talks to Mr. Cunningham about his son, and her plain decency breaks up a mob her fists never could.
Three: burying it. In Coco, Miguel's family has banned music, so his love of music feels like betraying the people he loves.
How the ability was hidden decides how your ending frees it. If staying away hid it, the ending puts your protagonist in the situation they avoided. If a substitute did, it brings a moment the substitute can't handle. If it was buried, The Lie has to lose its grip.
Name which of the three your protagonist uses. Then check that your ending undoes that one, and not another.
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.