In my younger and more vulnerable years
By saying that he was more vulnerable in his younger years, Nick is implying that he is still vulnerable, just not as vulnerable as he once was. He’s also hinting at his future development into a less naive, more disillusioned person.
Nick is writing at the end of the entire story, after experiencing the entire tragedy. He is looking back and making a comment that can' t be totally trusted. Nick proves throughout the novel to be an unreliable narrator, very unaware of his own weaknesses.
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