Today I read this post on Dear Polly, an advice column I read regularly, written by Heather Havrilesky: https://www.thecut.com/2019/08/ask-polly-i-cant-speak-to-anyone.html
While I think her response is great, I think she misses an important point. It's natural that she's missed this, because I haven't seen that a lot of people outside the mental health profession recognize this - that this level of anxiety that the letter writer is experiencing is WORK. When she says she has slow cognitive function and can't think of anything to say, it's because her brain is already using its supremely powerful processing capacity on all the thoughts that are driven by her social and other anxieties. Her brain is already very busy.
Social engagement, unsurprisingly, is also hard work. When you're already using your processing capacity on the worries that are driven by anxiety, it makes sense that little capacity will be left for social engagement.
It's a pretty common thing for people who suffer from anxiety to feel that they're not performing well in other areas of their lives, and they blame themselves, saying they're just stupid, or not good with people. But in reality, it's just that the mental resources they could be allocating to book or social or whatever-else smarts are being allocated to the anxiety-driven thoughts. There are ways to work through this!