"I'll feel better if I don't go"

You will. That's the difficulty. If cancelling made you feel worse straight away, the habit would never have formed.

Look closely at the sequence on Thursday afternoon. For four days the dread had been building. You sent the message, and within seconds the dread was gone. Few things in ordinary life remove a bad feeling that fast or that completely.

Anything that reliably removes a bad feeling gets repeated. This is one of the oldest findings in psychology, and you don't need a study to believe it, because you can see it in your own behaviour. It doesn't work through reasoning. At twenty to five you weren't weighing the evening's merits. Some part of you had learned, from every previous occasion, that the message makes the weight go away, and it wanted the weight gone.

The relief teaches a second thing, which is worse. When you escape something and feel better, your mind concludes that the thing you escaped was dangerous. It wouldn't have felt so good to get away otherwise. So each excuse confirms that leaving drinks are a threat, and the next invitation arrives already marked as one. What I've described here is interpretation, though it's close to how therapists who treat social anxiety explain avoidance.

Then there's the part of the sequence the habit never takes into account, which is Friday night. The relief lasted perhaps an hour. After it came a flat evening, a twinge of guilt about the invented problem at home, and the photo at eight o'clock. The habit is learned from the first hour, because that's when the feeling changes sharply. The cost comes in slowly over the following day. The cost is larger than the relief, but it's too spread out for the habit to learn from.

So "I'll feel better if I don't go" is true for about an hour, and it's false for the evening, the weekend, and the next invitation.

There's an honest objection here. Sometimes you cancel and the evening in is lovely, and you don't regret it at all. That happens. When it does, I'd guess the invitation was one you never wanted, and the refusal was an honest one that happened to arrive late. Chapter 15 is about those. The habit this book is about has a different pattern. You wanted to go, you didn't, and at eight o'clock you wished you had.