Acknowledging his "privileged life", he explains his position did not make him "immune to depression."
The 31-year-old goes on to write about the disease as "an absence of feelings ... only heart-thudding anxiety propelled me out of bed in the morning."
It's been just over a year since Middleton says he experienced "a progressive deterioration" and decided he needed to escape. So without telling anyone he packed his dogs into his car and drove to a remote area he has "loved since I was a child".
There he says he existed in total solitude, "eating from packets and trying to still the tumult in my mind."
Days earlier, Middleton says he'd accepted he was no longer able to cope with his condition and needed 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 writes.
He reveals he eventually allowed his GP to discuss his condition with his worried family, feeling he couldn't explain things in an objective way himself.
A year on, getting help has led him to work out coping strategies and while he would "never say I am cured of it" feels a "zest for life".
Middleton, who has recently been photographed with a new girlfriend holidaying in St Baths, says he eventually enjoyed going to therapy and always took along one of his five dogs for support.
In addition to sharing his battle with clinical depression, Middleton's column travels back to his childhood, his learning disabilities and the impact that had on his self-esteem growing up.
He has since been diagnosed with ADD and has begun to embrace his "impulsiveness" and "energy" as he's realised how they can be utilised positively with his "creative and individual mind"
He ends his article telling readers, "If I could leave you with just one thought, it would be this: 'It's okay not to be okay.'"
Read his full column here.
WHERE TO GET HELP:
If you are worried about your or someone else's mental health, the best place to get help is your GP or local mental health provider. However, if you or someone else is in danger or endangering others, call 111.
If you need to talk to someone, the following free helplines operate 24/7:
DEPRESSION HELPLINE: 0800 111 757
LIFELINE: 0800 543 354
NEED TO TALK? Call or text 1737
SAMARITANS: 0800 726 666
YOUTHLINE: 0800 376 633 or text 234
There are lots of places to get support. For others, click here.