Describing it as “an absence of feelings”, Middleton explains that his sister’s work with mental health charity Heads Together was partly what inspired him to come forward with his story.
"I know I’m richly blessed and live a privileged life. But it did not make me immune to depression," he writes.
"It is tricky to describe the condition. It is not merely sadness. It is an illness, a cancer of the mind."
Middleton's mental health reached such a low point by the end of 2017 that he felt unable to communicate with his loved ones, despite them expressing concern for him over his wellbeing.
"I also felt misunderstood; a complete failure," he writes. "I wouldn’t wish the sense of worthlessness and desperation, the isolation and loneliness on my worst enemy. I felt as if I was going crazy."
Middleton decided to travel to the Lake District with his five dogs in an effort to clear his mind.
It was at this time that he came to the realisation that he could no longer cope on his own and that he needed to seek out help.
"And this recognition led to a sort of calm: I knew if I accepted help there would be hope. It was a tiny spark of light in the darkness," he states.
After the majority of 2017 appeared to pass by Middleton "like a fog", he started seeing Dr Stephen Pereira, a consultant psychiatrist and cognitive behaviour therapy specialist.
It was through Middleton's consultations with Dr Pereira that he also received a diagnosis of Attention Deficit Disorder (ADD).
While Middleton was already aware that he is severely dyslexic, the news that he also had ADD helped him to gain a greater understanding of his character.
"It is the reason I have trouble focusing; why my mind wanders off into extravagant daydreams; why simple tasks such as making my bed assume the same enormity as filing my tax return," he writes.
Middleton views his ADD as a "gift" that's helped him to cultivate his "creativity and emotional intensity".
Nonetheless, he acknowledges that an earlier diagnosis would have benefited him greatly.
While he knows that his depression can't simply be "cured", Middleton is grateful that he has the opportunity to discuss mental health and break down the stigmas that surround it on a public platform, much like his sister and the Dukes of Cambridge and Sussex.
"I feel I have a duty to speak out, so I can help others who are suffering as I did," he writes.
"I am pleased I went through debilitating depression because I now have the skills to fight it."
You can contact mental health charity Mind for support by calling 0300 123 3393, emailing email@example.com or texting 86463.
Their lines are open from 9am until 6pm, Monday to Friday (except on bank holidays).