But as she slipped into unconsciousness, a man and his son, by chance, found her and called an ambulance.
"I ended up in a coma. When I woke up the doctors said that any longer and I would not have survived," she wrote.
Due to legal precautions, she was unable to name the exact location, but it was in the area of St Lukes on June 6, 2015, she wrote.
"I want to thank this man but I don't know who he is.
"I want him to know that I not only survived, but I fought and now help others for a living. He is a real life hero."
Thornton's remarkable story is told in her first book, Stop surviving, start fighting, which was published in March.
In May this year the mental health advocate returned to the spot where she'd tried to take her life five years earlier.
There she placed a sign, for anyone else in crisis.
"Please don't take your life today. The world is better with you in it," it read.
She returned to that place to change the memory it held into one of hope, she posted online.
"The girl who once stood here all those years ago was broken, unwell and couldn't see any way out. She was certain that the world was better without her.
"Little did she know that one day she would wake up excited for life. That she would know purpose, love and community.
"That one day the very thing that nearly claimed her life would be the thing she would use to help change the world.
"No matter how dark it feels right now, there is always hope."
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155
• Samaritans: 0800 726 666
• If it is an emergency and you feel like you or someone else is at risk, call 111.
READ MORE:
• Stop surviving, start fighting: Jazz Thornton's suicide story saving lives
• Jazz Thornton made 14 suicide attempts - now she stars in a documentary to help others
• 'It's very emotional': Heartwarming photo of Jazz Thornton with police officer who saved her life
• Jessica's Tree director Jazz Thornton invited to discuss suicide video at Buckingham Place