Maybe she will forget to text you. Did she tell you the date and time? I would just say after checking your calendar, you have a conflict that evening an won't be able to make it.
Unless your anxiety is such that you get physically sick at this sort of thing ... I'd just suck it up and go. You might have a great time! Especially if you already get along well with this woman.
But if it's really out of the question for you to go, when she contacts you I'd just say, "Thanks so much for the invitation. Unfortunately I won't be able to come after all. Have fun!"
you know... this post makes me realize how reserved I can be. Maybe I should break out of my shell.
I'm super introverted, so I totally understand not wanting to go. To something like this. Sometimes you do have to push yourself a bit and step outside your comfort zone. If all goes well, you end up making new friends and if not then you just felt awkward. Life goes on.