Oh derps! Anxiety is a cruel mistress. It pokes its ugly head whenever something big is happening in our lives. and replaces everything we should feel like ( excitement, happiness, etc ) with less than desirable emotions. I loved some of the advices I've seen you handing, mr Butter, and they're brilliant. You have the means and the knowledge to handle anxiety. Given more time, I'm positive you can wrestle it into submission.
As for your party, don't let the doubts get too heavy. All kinds of people need breaks from something. Your folks might not have noticed your absence, and you were very brave for going back in there. I would've done the same. Take a minute in the bathroom, put a smile on my face, REMIND MYSELF that this is a one-time occasion to have fun and go back. Sometimes it works, sometimes it doesn't, but if you give your brain enough of a kick to make it realize how improper anxiety is for that situation, you will be making progress.
I wish you the best of luck in the future. Here's to success and happiness