She explained she first heard the world's climate was changing when she was eight, and couldn't understand why so little was being done about it. By the time she was 11, she was very unhappy indeed. She stopped eating. She stopped growing, and she stopped speaking to almost everyone.
"It felt like I was the only one who cared about the climate and the ecological crisis," Thunberg told me. "My parents didn't care about it, my classmates didn't care about it, my relatives didn't care about this. I mean nobody I knew cared about this and I felt like I was the only one."
She was determined to change that and, starting with her family, she began her campaign to get the world to take action on climate change.
First, she persuaded her parents to stop eating meat and even got her mother, Malena Ernman, a famous opera singer whose career depends on travelling, to agree to stop flying.
Next came the climate strikes. One Friday just over a year ago (20 August 2018) Thunberg walked alone on to the steps of the Swedish parliament building holding her now-famous "Skolstrejk För Klimatet" - school strike for climate - sign. The rest is history. She rapidly became one of the most celebrated - and reviled - people in the world.
She has developed a network of contacts who advise her - including climate scientists and campaigners - but she's the one who calls the shots.
Those close to her confirm that she writes all her speeches. She told me she planned to use the two-week trip across the Atlantic to figure out what she was going to say at the UN Climate Summit.
"I will try and communicate the urgency and say that they are the ones in charge now. Show leadership!" she said.
She certainly did that on Monday when she accused world leaders of "stealing my dreams and my childhood", and warned them that "the eyes of all future generations are upon you. And, if you choose to fail us, I say: 'We will never forgive you'".
So, how about her mental health? And, while we're about it, does anyone have the right to comment on it? What's so impressive about Thunberg is how clear-headed she is on the urgent need for action in the light of the science.
Understandably, Thunberg particularly resents her mental capacity being called into question. "Being different is not an illness and the current, best available science is not opinions - it's facts", she said this week.
She acknowledges that she was very unhappy in the past, but says that championing the climate issue has helped lift her out of despair.
You bet your pale ass that's some Kim Jong-ill level of revisionism!