Final answer:
The correct word to fill in the blank is 'retract', meaning to take back or withdraw something said.
Step-by-step explanation:
When you realized you were wrong and decided to take back or withdraw everything you had said, the correct word to use is retract. Therefore, the completed sentence should read: When I realized I was wrong, I decided to retract everything I had said. This verb aligns with the context of the sentence, implying a withdrawal of a statement or accusation. The other options provided do not accurately fit the context of the sentence (petulant means showing sudden, impatient irritation; evince means to show clearly; invincible means incapable of being conquered; and protracted means extended in time or space).