Don't beat yourself up too bad.
I recently started clipping and using coupons myself in the last few weeks. It's not a process learned over night. And I'm assuming you're not trying to be one of those...people whose goal in couponing is to get out of the store spending as little money as you can (meaning well under $100 on lets say $200 worth of groceries).
My advice to you is this...make out your grocery list. Grab your coupons and see what coupons you can use that week in conjunction with your grocery list. It may not save you much...but over time, those savings DO add up. I find I save about $5 a week using coupons (not to mention the discount I get for bringing and using my own cloth bags, or the 5% I save using my target debit card as we primarily shop at Target). The toughest part for me using coupons is making sure I use them before they expire. But even then, if I don't, I try not to sweat it too much...and find that often times it's not something I use often enough to have just bought it to use the coupon.
A piece of advice for the future though...when you know you're having a party...plan ahead. If you have to buy meat or bread/buns, buy it a little at a time and freeze it. Same goes for any non-perishable items...and even plates, cups, napkins, plasticware. Then when it comes time to buy the perishable for the party, you don't freak about going over on the grocery bill...because you have bought when you can and frozen it or whatever!
Hope this helped! Have a great party.