I was a special adviser in a dying Government. To be really candid, I was in my mid 20s and I knew I was out of my depth.
So, when Sir Keir Starmer delivered his statement outside that same black door this morning, I felt a flicker of dread for the people still inside, because I know exactly what today looks like in there.
Watching a Government end from the inside is not what you imagine. It is quieter and slower than you think, and then all at once very fast.
The thing no one tells you is how human it gets. People cry. In stairwells, at desks, down a corridor where they think no one can hear them.
I've been in No 10 when a PM resigns. This is what it's really like - Oscar Reddrop
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Serious, formidable people who have given years, sleepless nights and days where their phones melt with constant WhatsApp messages.
This work simply stops mattering, and there is nothing harder to watch than talented people realising, in real time, that their effort has become pointless.
Then comes the paranoia. Phones turn facedown. Everyone is suddenly working out who is briefing what, who is already texting the next leader's people, and who is quietly clearing a desk and pretending they aren't.
Loyalty becomes performative and it inflates in value precisely as it drains away. You learn that the herd doesn't only move outside the building. It moves inside it, too, desk by desk.
Ultimately, with Boris, it was over when the Cabinet began to topple. Sajid went, then Rishi, and the holes in the hull were too big to patch.
But here is the part I only understood later. Boris fell with a record. Whatever else you say about Boris Johnson, he could slam his hand on the desk and point to things that existed: one of the fastest vaccine rollouts in the world, leadership on Ukraine when others hesitated, and a Brexit deadlock broken.
Even at the end, there was something to mourn. I don't know what the team in Downing Street have to mourn today.
I'm not sure they do either. Ask a Labour MP what this Government was for and watch them search for the answer. It isn't there, not because they're stupid, but because their leadership in No10 never told them.
A Government can survive being hated. It cannot survive not knowing itself. Ours ended with an inventory of what we'd lost: missed opportunities. This one ends questions it never managed to answer.
When it was my turn to go, it took less than two minutes. You hand your pass to the officer on the door, the same pass that an hour earlier had let you walk into the centre of the government.
No card, no clearance, no reason to be there. You leave by the back, out through the gates onto Horse Guards, because nobody walks out the front on a day like this.
And the strangest part is how normal everything is. Tourists, a bus, someone on their phone at lunch. The world hadn't noticed anything had changed.
No10 seems to make people lose touch with reality; it becomes a stupid game. I just really hope whoever ends up in there come September doesn’t make the same mistakes and can deliver for the great people of this country who deserve so much more.