I'm a long-time reader of these groups and very rarely post, but I couldn't go past this one. Hello, I am in my thirties and feel like a young, clueless idiot pretty much 24/7 :-) I also had postnatal anxiety, and also postnatal depression, so I understand how hard it can be just to get out the door.
I'm not usually someone who finds it difficult to make small talk- but I felt much the same as you describe at the group I attended. In my case there were no specific incidents like the awful one you had but I felt awkward and useless, and found that the many graphic descriptions of complications in labour discussed by other mothers tended to give me panic attacks at the time. To be fair, I needed to get out and about, and I needed friends, but I wasn't ready for mother and baby playgroups until my baby was nearly a year old.
It is different for everyone, but I found that going along to baby massage classes was more helpful- a more formalised format, focused upon the baby, with the odd bit of chatting if you felt like it in between. I didn't do baby yoga, but maybe that is worth a go too? Also, do you think you might be able to find a group for mums with postnatal anxiety? Otherwise, if you can find one or two mums to meet up with for a coffee, it might be a bit easier than having to deal with a group of strangers.
By the way, cailindana, have you ever considered that at least some of the mothers who you found difficult to communicate with might have been suffering from postnatal depression? It is really hard to be smiley and 'make the effort' with people when you are feeling anxious or depressed. I also really disagree that dealing with horrible people is 'life' and that you just have to get on with it. New mums at these groups, and some not-so-new mums, need reassurance and friendliness. I think if there are people being rude, cliquey and unwelcoming at a group, someone should have the guts to call them on their bad manners.
I am sure it was really difficult to deal with me when I was at my worst but when people were kind and welcoming it made a huge impression on me. bluejeanswhiteshirt, I hope you find a nicer group soon!