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If you had to choose, which one would you consider more natural?

"Stop calling me that. I may be disabled, but I will not let it define me."

"Stop calling me that. I may be disabled, but I won't let it define me."

"Stop calling me that. I may be disabled, but I'm not going to let it define me."

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Those are all natural. "will not" seems more formal or more emphatic than the other two.