Hey there 427,
I think that you're definitely right to not freak out about what happened. The more upset you are, the more guilt you'll have, the more anxious you'll become, AND then the resignation will settle right back into your thought process. Avoid the comfortable self loathing.
Yeah, so you drank. Yeah, bummer for steady sobriety. But, that doesn't mean you can't go back to being a non-drinker immediately, ya know? Remember how good and clean you felt during the past couple of weeks and remember why you quit drinking in the first place. If you approach your drinking through a cost/benefit analysis framework, I'm sure you'll come to your own truth regarding drinking.
You know something that helps me? A worry stone - a rock that is slim and smooth. I held a stone in my hand while I came to my own truth about my drinking, and I still keep it in my pocket. The subtle weight of the stone reminds me of my new lifestyle.
Any way, take care and keep us posted.