Like you, I stress out for parties too. My family hates the mood I get into when shopping, cleaning, preparing food, etc. for parties. I'm always worried I'm going to forget something! The people I imagine who are critiquing my home and cleaning standards, aren't really here to do that. But I think they are! Silly thoughts right? The one I stressed out so much about last Dec. for our 30+ family xmas day gathering, had trouble opening a bottle of wine, so much so that it sprayed all over her satin blouse, the cork came apart and bits/pieces were floating in the bottle. This happened just as she arrived! She laughed, I laughed and I thought to myself - what the heck am I so stressed out over? LOL
Sometimes you need a small reminder like that relax.
Now here's a trick - imagine it's not today but tomorrow. Imagine how you will feel tomorrow - calm, relieved the party is over, you'll have a clean home (aside from the clean up of cups, plates, garbage, etc.). You can RELAX! So try your best to put your mind there and not on today.
Projecting forward is something I did when in L & D with my two children, I just pretended it was a day, week, month later and the pain wouldn't be going on. I used this trick after recovering from my 5 hour colon resection surgery for rectal cancer (9+ years ago). I would tell myself, 1 week from now I'll be feeling better, 1 month from now....and so forth.
Not to take away from the actual moment but just imagining the calm you'll feel tomorrow, might bring about calm nerves today, when you need them!
I'm sure your party will be a huge success. Just don't drink or eat. Sip, nibble, make it look like you're drinking and eating, but you're not....LOL That's how I get thru a party!