At first she put it down to the change of environment.
"I didn't have any support and was finding it really hard to deal with the pressures of society, social media and a busy live," she said.
"With the typical Kiwi attitude I pushed myself and was in denial that anything was wrong. I kept telling myself I didn't need help but at the same time I was confused and asking myself 'what is this'.
"I kept driving myself. In my head that's what success looked like. Keep going and it will be okay."
However, for Catie, it wasn't okay.
"I crashed - lost control and had to go home."
Catie still didn't know what was wrong with her but she knew something wasn't right.
With the support of her family and friends and help from professionals she was diagnosed with bipolar disorder just before she turned 19.
"It's been a long road to recovery, more than four years, with some highs and lows, working with professionals through manic depression, but life for me is so much better now."
During her recovery a psychologist gave her a book and told her to write down how she was feeling.
"That really inspired me. I started to write poems and discovered a power in me which needed to be let out on paper. I didn't plan to write a book but when I was better and looked back at what I had written I thought my collection of poems was pretty cool and that maybe I could turn them into a learning tool to help others going through similar experiences."
So Catie put her poems in order and sent them off to a publisher.
"I didn't tell anyone. I didn't consider myself a writer. Although I am creative I would describe myself as having a science brain."
She was very excited when the publishers got in touch with her but also felt vulnerable putting "myself out there".
Catie's book Her Patient Fight, poems of a journey, hardship to hope, was released on September 8.
Her poetry is insightful, moving and inspiring.
"I hope this book inspires people going through mental illness or any adversity in their life. You can come out the other side."
She says her family and several close friends have been amazing throughout this entire journey.
"They have picked up the pieces, taken me to appointments and I feel so lucky to have them. It takes a lot of people to fight a mental illness. It wasn't just my fight."
She says her message to anyone going through mental illness is to be patient.
"It's a journey you are going through. Don't give yourself a hard time. Rest is okay, just keep going forward, don't give up. Life will get better."
Catie is now working in a support role and is loving it.
"It brings me so much purpose."
Her Patient Fight
Poems of a journey, hardship to hope
By Catie Nettlingham
Mary Egan Publishing, $30
Available from Wardini bookshops and can be ordered in all other bookshops.