Cindy Wachenheim was someone people didn’t think they had to worry about. She was a levelheaded lawyer working for the State Supreme Court, a favorite aunt who got down on the floor to play with her nieces and nephews, and, finally, in her 40s, the mother she had long dreamed of becoming.

But when her baby was a few months old, she became obsessed with the idea that she had caused him irrevocable brain damage. Nothing could shake her from that certainty, not even repeated assurances from doctors that he was normal.

“I love him so much, but it’s obviously a terrible kind of love,” she agonized in a 13-page handwritten note. “It’s a love where I can’t bear knowing he is going to suffer physically and mentally/emotionally for much of his life.”

On March 13, 2013, Ms. Wachenheim, 44, strapped her 10-month-old son to her chest in a baby carrier and leapt to her death from the eighth-floor window of her Harlem apartment. “I became so low,” she wrote in the 13-page outpouring shortly before she jumped, “thinking that if I had unknowingly caused brain damage to my beautiful, precious baby, I didn’t want to live.”