This is a great thread!
I sort of disagree with you Rossanne - in my opinion, the self-promoters have more credibility and are less vulnerable because they're in control of their public image, and trying to get the right people along to their shows. Of course, it's possible that they're not very good at this, and like you say, there will always be someone negative in the audience. But, if you've been marketed by someone else, who knows what they're saying about you, or the conversations they're starting. You may think you know what's out there, but do you really? I think this is especially true for the more successful acts out there, whose record companies subscribe to the 'no news is bad news' model of marketing.
Feeling vulnerable at gigs is natural. It's a feeling that may never go away, but it's one you learn to manage. Recently I did a charity gig for a local country band, whose songwriter really likes my songs. The audience were all there to see him and had never heard of me. I played my set, and everyone politely listened and clapped. It was very uncomfortable. But then, after 4 hours (it was a long night with a guest speaker) I was approached by half a dozen separate people who said I played well, or had a nice voice, or that they enjoyed it. Came as a big surprise to me!
Everyone is different, and nobody likes negative feedback, but you're going to get it sooner or later. You can't please everyone, and there's usually drink involved at gigs. Try to remember the reason you're playing and they're listening. It's because you're a good songwriter, singer and musician, and they're none of these things. They might be terrible at their job! I've been listening to your songs from your website, and it's obvious that you're good enough to know how good you are, and where there's room for improvement.
At some point, you have to develop the skill of only listening to criticism from those qualified to give it. The majority of people will listen with an open mind, and enjoy hearing good music played well. As for the comment about the kittens... maybe you could incorporate that into your act the next time you play there?
In all seriousness, this is what we open ourselves to when we decide to play music in public. No job is perfect, and there are down-sides to everything. Ask yourself this: which is harder to deal with: a gig where only 5 people turn up (happened to me on Friday) or a gig where someone says something ridiculous and slightly upsetting, but you have an audience at least?
I don't have the answers, and I really empathise. In fact, if I'm honest, I never played my own material at gigs until last year (I'm 35 now) just because I was too scared of people not liking it. You're doing brilliantly! Have a cake to celebrate