"I never know what to say"
In the imagined version of Friday, someone turns to you and asks what you've been up to, and your mind goes white. This does sometimes happen. It's worth understanding why, because the usual explanation is wrong. You do have things to say.
When you're nervous in company, your attention goes inward. You monitor how you're standing and what your face is doing. You check whether your voice sounds odd and whether that last remark was stupid. Therapists call this self-focused attention. Freda McManus, Catarina Sacadura and David Clark tested it directly. In one study they had some people focus on themselves and use their usual protective habits in a social task, while others focused outward and dropped those habits. The inward, self-protecting version came off worse in a number of ways. I've only read the summary, which doesn't list them all.
The reason isn't mysterious. A conversation takes attention. You have to hear what the other person said, take in what they meant, and find something that connects to it. If most of your attention is on your own face, you miss what they said, and then you really don't have anything to say, because a reply has to be a reply to something. So when your mind goes white, it's because your attention was on yourself and you didn't catch what was said.
The NHS's self-help advice for social anxiety includes this very point. It says to try to focus on what people are saying rather than just assuming the worst. It sounds too simple to help, but I think it helps more than anything else in this book.
It helps to notice what conversation at a leaving do consists of. It isn't a performance of wit. Mostly it's questions and listening. People ask where someone is going next, how they found the job, and what they'll do with the week off. Anyone who asks a real question and then listens to the answer is good company, and is remembered as such. People with this habit are often better at the listening half than the confident talkers are, because they're not waiting for their turn to talk.
So you don't need things to say. It's enough to have two or three questions in your pocket, which chapter 16 will help you prepare. You also need to bring your attention back to the other person each time it drifts to your own face. It will drift, and you bring it back again. After ten minutes of that you'll usually find you're in a conversation.