I'm not sure how much of that has to do with giving up drinking, because I tend to "find things to worry about" anyway. It's not necessarily out of a need to be dismal or cantankerous. Maybe I need to find problems to solve or at least grapple with. When I think of the experience of giving up drinking, it makes sense to me that one of the things people go through is this sudden fear out of the blue. I suppose it has to do with getting used to contending with issues with your own mental appartus -and with no accessories, like substances to abuse.
The free-floating anxiety being deferred and then released later is interesting.