Doesn't sound like a failure to me, more like a lesson as long as you take it on board. What happened was the offer of a drink came from left field and you had no advance plan to handle it. So many fatal first drinks occur that way.
In my experience it is impossible for an alcoholic of my type to anticipate every situation and have a defence in place every time. It is one of the basics that AA is about, finding a 24/7 defence against the first drink. It goes beyond our human ability to be on guard all the time. At certain times we have no effective defence against the first drink. That defense must come from a higher power.
A friend of mine was at a family function. He was about three months sober. Everything was going well, not a care in the world. Someone offered him a drink. Without a thought he said yeah thanks and almost drank it before his sister noticed and knocked out of his hand. Saved, but not by himself. He was shocked at how close he had come, and how easily it could have happened.
Strange as it may seem, my recovery really kicked off when I gave up trying to not drink, and caught onto the idea that this higher power stuff is the important part of the program, and began trying to develop the relationship that would give me the 24/7 defense. That was 1980, I was 22, and I havnt needed to take a drink since.