Thanks to a recent spate of high-profile suicides, coupled with newly released data from the Centers for Disease Control and Prevention revealing the national suicide rate to be at a 30-year high, many people are contemplating mental well-being. In 2015, Poppy Jamie was hosting Snapchat’s first talk show, “Pillow Talk With Poppy,” when she began to think there was a particular malaise plaguing millennials. So she began incubating a new “brain health” app called Happy Not Perfect. “I would get hundreds and hundreds of messages from all these young people around the world,” said Ms. Jamie over tea at Soho House in West Hollywood not long ago, “and what I realized was that everybody was saying the same thing: ‘I’m so stressed.’ It didn’t matter what country, gender, age. It was unanimous.” According to the C.D.C., suicide is now the second-leading cause of death among 15-to-34-year-olds. Ms. Jamie, a graduate of the London School of Economics, has spent the past three years working wit