Before I had PND, I already had a psychotic illness. I’ve had it since I was about ten, existing in the gaps between reality and unreality. It’s confusing and distressing. I often go out in my car and forget how to drive and also what a car is and why streets exist. Sometimes I panic to the point of total dissociation, which is like seeing yourself replayed in a movie you can’t remember shooting.

I had presumed, being mental already, that I’d get postnatal depression. How could I avoid it? I could barely go to the supermarket without crying (why are there so many kinds of yoghurt for the love of God!). While I was pregnant, I’d already conceded defeat, getting my black dog a new bed and a fancy ceramic bowl.

But I had got it all wrong. PND didn’t just blend into the depression I already had. I didn’t just become “more depressed”. It was a whole distinct illness with its own symptoms and its own treatments.

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One in seven women experience PND. Doctors aren’t exactly sure why it happens, but it’s likely a combination of massive waves of hormones and the shock of growing and then becoming responsible for a whole human person. Babies cry a lot, and loudly. You can walk right to the end of your driveway and still hear them. But also, your body is all different and parts of it hurt and no one understands and why does your partner get to go to work while you have to stay home and wait until your most rudimentary life-sustaining skills are required? DON’T RING THE DOORBELL OH CHRIST NOW THE BABY IS AWAKE. WILL MY BOOBS EVER STOP LEAKING?

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Over the past three decades, I’ve come to know my brain through trial and error. I’ve pushed and challenged it, understood its limitations and learned to sit uncomfortably with it. By the time I had my baby, at 20, I didn’t like my brain, but I understood it. Sort of.

PND was different.

PND was like having a neurotic couch-surfing aunt come to stay.

I can’t remember the first time I felt it, but I remember it was new. I’d had this other illness my whole life, and now I had something else. It approached me very obviously, very boldly. I noticed the same types of symptoms coming to life quite separately from the ones I already had.

Did you know there are different ways to feel sadness? I didn’t. Suddenly I was regular sad and this foreign, alarming sad. I was two kinds of anxious, two kinds of meaningless. I had my usual existential dread and another, more acute despair. I felt it like two rivers: a rushing rapid above the ground that would pummel me to death right now, today; and the quiet flow in an underwater cave that would slowly suffocate me.

As I tried to fight one, the other attacked me. As motherhood overwhelmed me, I could “see” the PND, totally discrete.

When my daughter was a few weeks old I went to the maternal and child health nurse for a regular check-up. I knew it should have been nice to walk there, pushing my imaginary doll past the canal. The fresh air was objectively pleasant, the winter streets pretty in theory. I sat at the nurse’s desk and she humoured me by pretending to weigh the baby I had invented in my mind.

Afterwards, she looked right at me. No one had looked right at me for weeks, no one had been able to focus on the shadow person I’d become. She stared and frowned and then she said, “You look like your heart hurts.” I will never forget the way she said it, so plainly. Like she could see the PND as well, like it had pulled up a chair next to me. I said, “It does.” Then I cried through every tissue she had and some she got from another room.

I tried to explain the way my old illness and I had been taken prisoner by this newcomer. Regular anxiety told me it was dangerous to go outside. PND told me I could never be a good mother. Regular depression told me to lie on the couch all day. PND told me to put a pillow over my baby. Regular psychosis made me afraid to look at myself in the mirror. PND told me I had invented my baby and I couldn’t connect with her emotionally because she was a delusion.

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After the nurse visit, my doctor prescribed new medication and referred me to a new psychiatrist. We were no longer treating the illness I already had but trying to starve out the new one. For 10 months, we fought it. Every day was like sitting at a table with two people I hated. By my daughter’s first birthday I was wrung out, beaten blue by this stranger, but on my way to being well again.

The great deceit of postnatal depression is that it makes you think you know it. I felt like my PND had been inevitable. My brain was so suggestible, open to a whole smorgasbord of mental illness symptoms. I knew I would have PND, I knew, I knew.

But when my second daughter was born two years later, it never arrived. I held my baby and listened to the sweet warm coo of her breath and I only had my regular brain funk. I was depressed, but just one kind. One kind of flat, one kind of despairing. I was anxious, but in the way I’d spent my whole life learning how to be.

• Anna Spargo-Ryan is the Melbourne based author of The Gulf and The Paper House, and winner of the 2016 Horne prize

In Australia, support services for postnatal depression can be reached at beyondblue 1300 22 4636 and Healthdirect 1800 882 436. In the UK, the Association for Post Natal Illness can be reached at 0207 386 0868, or contact Pandas Foundation on 0843 28 98 401. In the US, Postpartum Support International is available at 1 800 944 4773.