"It's getting harder, so I must be getting worse"
A year ago you went to most things, with some nerves. Now a drink round the corner feels like an ordeal. It's natural to read that as a change in you. You conclude that your nerves are getting worse and you're less able than you were, and that perhaps you're simply not built for this.
I think there's a better explanation, and it's in this book's subtitle. Each refusal makes the next invitation harder. Here are four ways it does that, and they're my own reasoning except where I name a study.
First, the relief marks the event as dangerous, as the last chapter described. The mark accumulates. After ten escapes, social evenings carry ten layers of it.
Second, you never get the evidence that would correct the picture. In your imagination the leaving drinks went badly. You stood at the edge, your mind went blank and your face went red. Because you didn't go, that imagined version is the only one you have. It sits in your memory almost as if it had happened. If you'd gone, there'd have been an actual evening to set against it, and chapter 5 gives reasons to think actual evenings are usually milder than imagined ones. If you never go, the imagined version is never tested against anything.
Third, you get out of practice. Easy conversation is partly a skill, and skills fade without use. If the last time you made small talk with a half-stranger was four months ago, it will be stiffer than it was when you did it weekly. You notice the stiffness and take it as proof that you're bad at this. It only shows that you haven't done it lately.
Fourth, the group moves on without you. Every evening you miss produces jokes you weren't there for, stories you haven't heard, and a bit more ease between the people who were there. When you do turn up, you're slightly more of an outsider than you were, and that makes the evening harder and the next refusal more likely.
Rachman and colleagues, studying what people do after an awkward social event, found something that fits. People who went over an event more afterwards also reported more avoidance of similar situations. Avoidance and the painful going-over went together. Chapter 13 comes back to that.
All four of those work in reverse as well. An evening attended leaves one less layer of danger and an actual memory to draw on. It gives you some practice and a shared joke. So the difficulty you feel is mostly a measure of how much avoiding you've done recently, and that can change.