"It calms me down"

It does. Your hand on the locked handle, and the doubt goes, and for a moment you're fine. That's not imaginary, and it's the whole reason the habit exists. Anything that removes an unpleasant feeling at once will be done again.

But look at how long the calm lasts. Back in the car, the seatbelt on, and already the picture of the handle is softening. By the lights it's the straighteners. The relief bought you ninety seconds. Then the doubt is back, and it has grown, because the picture it feeds on is fainter than it was.

There's a pattern here that's worth naming, and it's the same one that runs through every habit of reassurance. Something makes you anxious. You do a thing that relieves the anxiety for a short time. The relief teaches you to do the thing. And because the thing doesn't touch the cause, the anxiety returns, and the thing is done again, and each time the relief is shorter and the return is quicker. Rachman's loop from chapter 7 is this pattern applied to checking, with the extra twist that the checking damages the memory it's meant to confirm.

What I'd ask you to notice is the difference between calm and certainty. The recheck gives calm, briefly. It can't give certainty, because certainty of the kind you want, a guarantee that nothing has been left, isn't on offer from any number of looks. The door could be locked and a window open. The hob could be off and the toaster plugged in. There's always another thing, and the doubt, once it's learned that checking calms it, will find one. This is why people who check the door come to check the windows, and then the taps, and then the car. The calm is portable. It goes wherever the doubt goes, and the doubt goes wherever the calm has been.

So the calm is real, short, and expensive, and it's the thing you have to give up to end the habit. Not the safety. The safety was the once. What has to go is the ninety seconds of relief from the recheck, and in its place there'll be a few minutes of doubt that you sit through, which chapter 18 describes. The doubt will pass on its own, and each time it does, the next one is smaller. That's the trade. Ninety seconds of relief that grows the doubt, against a few minutes of doubt that shrinks it.