Hi Michael,
Welcome to the Dharma Overground.
Question - Therefore, I wonder if this anxiety is related to being in the 3rd stage of insight, before crossing A&P, the fourth. Any thoughts appreciated. (I recognize also that I am not seeking any psychological or medical advice, of course! I may have underlying tendencies to social anxiety, which are just being exaccerbated by the meditation, temporarily.)
Great. I am glad you wrote, "Any thoughts appreciated."
My first thought is: cardiovascular exercise, such as timed swimming and elliptical workouts--increasing weekly in rate, a nice gradual healthy practice. Consider adding weights and yoga.
This is a real cross-training dedication: really allocating two hours before work (exercise, clean-up, eat, re-hydrate), and if you can afford it, consider getting some personal training, too. The trainer can safely help a person exhaust themselves, and going into work after a healthy workout is likely to cause the mind to be much less anxious. Furthermore, as things are cause-and-effect, the less anxious mind can interact with colleagues differently, start to grow safety and enjoyment. :]
As well, with exercise calming the mind, meditation can develop with better perspective and new terrain.
This level and frequency of exercise doesn't need to go on forever, but it is well worth a month-long dedicated trial, in my opinion and experience. It's important to stay friendly with oneself: not push too hard, not hurt oneself, but to really apply good training and effort. Treat oneself kindly, but make a good effort. A trainer can really help prevent injury and help one push. One week even with a trainer is insightful (I am not a trainer; this is not a sales pitch : )
There was a time when I would also have my just-outta-bed coffee with a vitamin B complex, and then I'd have another vitamin B complex after lunch. I was cautious to not consume too much coffee in the afternoon (one cup at 4 p.m. ish): coffee can really exacerbate anxiety/stress when the mind is already disposed that way.
What do you think?
Related question - would it be possible that I break past some of these symptoms by crossing A&P. I wonder if I should take a bit of time off work, away from the social situations that stress me, and really work on my breath meditation/concentration to try to move on, have an A&P experience, and move away from this stage.
Everyone is different, but I think there may be some consensus that what comes after A&P can be harder. The mind is said to enter the the so-called dark night stages (dukkha nanas). Maybe you are there? Anyway, it may be a harder development (aka: bhavana, meditation) phase simply because a person may not know the development is coming. So, for example, if I had known the buddhist framework for mental events at age 15, I'd like to think I'd have known how to skillfully continue. ;) Probably not.
So I'd prioritize a good sensible and sustained exercise effort now: consider having someone get you started right, some trainer who's recommended and who you basically like/feel good around (because it's normal to resent them a lot in the middle of a hard work out).
I know I did not speak much to meditation, but I am offering you the consideration that I think are most useful for what you've posted and in my experience.
Good luck.