Donald Trump’s “worst nightmare” is coming true, his niece said Monday night. Mary Trump, an outspoken critic of the former president, told subscribers of her newsletter, “The Good In Us” on Monday night that recent polls show her uncle — “the old, white, racist” is being beaten by a “young, strong Black woman.” Unsurprisingly, she added, he isn’t taking it well. That fact sticks in his craw and has contributed to his recent outbursts attacking Black journalists, women and Georgia officials, as well as congratulating Russian President Vladimir Putin on last week’s prisoner swap. Want more brea…