There was a moment, waiting for Prince Harry to arrive, that I had to pinch myself. It wasn't so much that I was about to interview a senior royal, though it would be a lie to say that I wasn't massively overexcited about getting to spend time in a small room with the most eligible bachelor on the planet.
It was more what I was about to interview that senior royal about.
In the bland, sanitised landscape that we find ourselves in, where the high-profile trot out pre-checked platitudes cooked up by public relations people and provocateurs respond by saying things simply to wind people up, it has become increasingly hard to get anyone to say anything genuinely interesting or heartfelt.
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And yet here I was, twiddling my thumbs in Kensington Palace, waiting for a member of the Royal family to talk to me about his mental health.
Was this really happening? I mean really really?
Were we finally living in an era where even a representative of one of the most buttoned-up, traditional institutions on the planet might feel able to talk about the troubles in his head?
To call it a watershed moment might sound dramatic. But not to anyone who has ever had to suffer the complexities of mental illness in stifled, suffocating silence.
I've told this story a fair few times before so will keep it as brief as possible, you're not reading this to hear about me after all.
When I was 12, and the only things I should have been worrying about were boys and periods and quadratic equations, I woke up one morning convinced I was dying of Aids.
I had not so much as shared a kiss with anyone by this point. My drug problem was at least a decade off.
But all the rational thinking in the world, and as time went on, I would become less capable of it, was useless when pitched against the voice in my head that told me I had weeks to live.
I stopped hugging or touching my family and friends so that I didn't infect them. I hid my toothbrush under my pillow. I tried to scrub all the disease away from my skin, leaving it red raw and bleeding.
For weeks at a time, I was too frightened to leave the house.
Then, at 17, I became convinced that I had killed someone and blanked it out in shock.
Suddenly entire swathes of vocabulary were out of bounds to me. I could neither read nor say words such as "rape", "murder", "death", "paedophile", "killer", "porn", "sex"... and on and on it went.
If I saw any of these words, I had to say several positive ones in my head because ... well, I don't know what. I would become a serial killer, a paedophile, be raped?
I started saying phrases to keep my family alive. If I thought I had got them wrong I would spend hours repeating them until they were perfect. They never were.
I was diagnosed with OCD, an illness I thought was the preserve of celebrities and clean freaks, an illness I would soon discover to be horribly debilitating, and sometimes disabling.
But mental illness was still not taken that seriously back then, and so after shuffling from therapist to therapist and popping some antidepressants, I would fall back into what I now refer to as my ''old'' ways: cocaine, bulimia, sex.
(''Old'' is somewhat misleading: as anyone in recovery knows, old ways are very good at making themselves appear fresh at any given moment).
It wasn't until I was 34 - 34! - that I finally got proper help and, shortly after, I wrote about my experiences for the first time. It was, bar getting pregnant and marrying my husband, one of the best things I have ever done. (To be fair, the list of good things I had done were, until that point, vanishingly small).
Hundreds of Telegraph readers wrote to me with their well wishes, but most importantly their stories. All of them were putting their hands up and saying "me too!" if not OCD, then some other form of mental illness. I realised then how completely and utterly normal it was to feel weird.
I wrote a book of my experiences, Mad Girl ("It's going to be an upbeat book about depression!" I told my editor cheerfully, before promptly suffering another breakdown.)
In the depths of despair once more, I started running, albeit briefly, just until I looked like I had been in the glare of a nuclear bomb. On one of these runs I was listening to a podcast about Carson McCullers, author of The Heart Is A Lonely Hunter.
McCullers made several attempts at suicide and died of alcoholism at 50.
There was some archive audio footage in which she announced "sometimes it feels like everyone is part of a 'We' except for me".
I'd like to say it stopped me in my tracks, but probably I was just out of breath. But I did find it profoundly affecting.
I went home, and decided to find Carson's "We" by using social media to set up a walk in central London for people who felt they had issues with their mental health ("But what if a load of nutters turn up?" asked my husband. "That's the point, darling, that's the point!")
At around the same time I and a small group of mad girls and boys were setting up Mental Health Mates, it's now in almost 30 locations across the UK, it was announced that Heads Together would be the official charity of the Virgin London Marathon 2017. That takes place next week. I don't know if I've mentioned, but I will be running it.
It felt like a sign - I had to take part in the world's first mental health marathon. What I couldn't have known was what an incredible, life-changing experience the journey to the start line has been.
Who cares what happens over the 26.2-mile course, given everything that has been achieved in the run up to it?
Heads Together was set up by the Duke and Duchess of Cambridge and Prince Harry to change the conversation about mental health.
But actually, it has created a conversation about mental health, one in which everyone from footballers to new mums has found themselves publicly talking about the stuff in their heads.
It has been the most wonderful thing to witness and the most extraordinary thing to be part of, this process of people turning huge negatives into massive positives.
I was over the moon when Heads Together became one of the Telegraph's Christmas charities.
The response we have had from readers has been so phenomenal that at the beginning of this year, in a soulless meeting room in Victoria, a group of senior Telegraph executives (and me), made a commitment to create a permanent area on the Telegraph's website dedicated to everything related to mental health.
Changing Minds is the result of that: A one-stop shop for advice, articles and a comprehensive list of crisis numbers.
Mad World is my little contribution to it, a podcast that I hope will allow people to talk about their mental health in much the same way in which we do the football scores down the pub.
The medium seemed so perfect to me: warm, intimate, in your head. I thought that maybe I might manage to bag a Professor Green or a Stephen Fry.
Then, at a training day for Heads Together runners last February, I found myself being interviewed with the broadcaster Sian Williams about why we were doing the marathon.
Sian, who has done several before, became emotional as she spoke about her nervousness at running for the first time after recovering from breast cancer.
"Don't make me cry!" she pleaded. "There are cameras on me!"
She hadn't noticed the fifth in line to the throne, who was standing to the side of one of the crew, watching us talk.
"I think Prince Harry should give you a hug," I beamed, somewhat insanely, beckoning him over. He did as he was told, before engaging us in a conversation about depressed people being the best marathon runners.
On the way home I was overwhelmed, not for the first time, with the desire to do something mad.
I decided to ask if Prince Harry would be the first guest. What did I have to lose?
I called up Kensington Palace and put in the request. A couple of weeks later I got the news that His Royal Highness would be delighted to. I had a little cry at my desk.
I had no idea, when I arrived at Kensington Palace, that Prince Harry was going to be so candid.
I thought that he might perhaps talk obliquely about the importance of discussing mental health.
Instead, it was like having a cup of tea with a friend who had been through a particularly difficult time, and lived to tell the tale.
He did so movingly and articulately. There was no sense that he was inviting pity. He was simply telling it like it is, like it had been. He did all of this while managing to be funny. I got the sense that Prince Harry has precious little time for those who wallow in sad music.
I think this interview is special not because it's a scoop or an exclusive. I don't think this interview is special because I happened to do it. I think it is special because in Britain, we don't talk about our feelings.
We have bitten our lips, slapped on rictus grins, kept buggering on.
It has always been a sign of strength and dignity to keep it all inside, and our Royal family have always been the embodiment of that, God bless them.
But Prince Harry just redefined strength and dignity for a new generation. He has shown the world that talking about your problems is nothing to be ashamed of - that actually, it is something to be positively encouraged. And I can think of no more fitting tribute to his mother than that.
• Bryony Gordon is a journalist for The Daily Telegraph UK.
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.