I don’t think he had ever really believed in its existence before.
The Daisy of his past didn’t have children and was quite likely a virgin when he met her. The fact that she has a daughter throws her relationship with Tom in Gatsby’s face, once again showing him that he has indeed lost five years, and that Daisy has indeed changed. And although he will never accept it, he can’t rewrite the past and the child is proof of that.
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