Hi Horn,
No need for shame. It was a moment of insanity over which you had no control. At certain times the alcoholic has no effective mental defense against the first drink. The sane thought that should have stopped you just didn't come. Instead the insane idea that as it was a happy occasion, a drink would be alright, won the day.
The lesson is that for some of us, we need to find an effective defense for those moments. I found one in AA. I could no more think my way out of a drink than I could think my way out of a headache. That is what is meant by powerless.