How to turn down an invite ( if I'm invited)?

I don’t go to any nights out with work, I never have. They have now stopped asking me unless it is for a big occasion. Well, there is a ‘big’ occasion coming up. Someone is getting married abroad and she is having a party for when they return. I think they will ask me to go, and she is well liked by the others, so not sure this will go smoothly.
The last time I didn’t go to something, I was accused of being Bulimic and a few of them were pretty mean to me.
I have social anxiety, and one of the girls does know this, although I don’t think she gets it.
I really don’t want to go to her party and it is worrying me a little, even though it isn’t until September.