Start by taking care of your body and mind for a day or so.
Don't beat yourself up, but don't minimize what you did either. For some reason you chose to do something that you rationally know is unhealthy for you. Somehow you overruled or ignored your conscious awareness that the costs of drinking outweigh any possible benefits, that you are an unlikely person to drink moderately based on your past behavior.
You've reopened some behavioral pathways in your brain that had begun to fade into distant memory. So for a little while urges are likely to be strong again, and your self-image is a little shot so your refusal skills (that's really what we're learning when we quit drinking) are going to be weaker than they were.
I'd suggest doing something every day to reinforce your commitment to abstinence. This is an opportunity to review what was going on when the 'lapse' started. And it's an important time to plan for future situations like the one you just went through.
Ask yourself some questions.
What were you thinking? No, I don't mean this the way your mother used to say it to you. I mean, what were you thinking during the moment when you started the process? Your thoughts and beliefs at the moment of decision: at the store, at the restaurant with your friends?
What were you feeling? Look carefully at the difference between thoughts and feelings. One is rational, the other emotional. Both lead to drinking. We can learn to counter the thoughts, and we can learn to manage the emotions. Were you feeling stressed? Anxious? Dejected? Angry? Giddy?
By the way, how are you feeling now? Be careful about rating yourself, about secondary upset (being upset about being upset), and watch out for that 'I give up -- might as well' thought process. You are not your behavior. CBT concept: USA (unconditional self-acceptance). And, if people around you seem to lead to anger and frustration, and thence to drinking: UOA (unconditional 'other' acceptance). Need a link to articles about USA and UOA?
What were you doing and where were you doing it? (You can ask this in your best Jack Webb imitation voice.) There's a psychological part of the process; we'll get to that in a moment. But there's a physical part, too. People who quit drinking usually don't have alcohol in the house, don't buy it for any reason, don't go where it's being served until they have considerable confidence in their refusal skills.
What did you do? I mean specifically: what was the behavior? 'I got mad so I went to the store and bought some wine'. Try not to describe yourself as a loser, as a failure, as lacking in character. Be accurate and describe the behavior. See if you can do it while avoiding 'good, bad, shouldn't, can't, always, never.' That can be difficult, eh? But it's the behavior -- not you -- that's the problem. 'I chose to drink, I have difficulty stopping when I start, and drinking is unhealthy behavior for me'.
What are you going to do next time? Look at the activating event, or the moment when your decision-making process tipped over in favor of drinking. That's the mood, the belief, and the thinking you'll want to plan for. CBT Recovery tool: do an ABC on the activating event (A) and the belief (B) that you were acting on which led to the condition (C) you found yourself in when you chose to drink. Then dispute (D) the belief, and try to get to an effective (E) new condition. Need a link to an ABC article?
What are you going to do now? CBT tool: try updating your CBA (cost-benefit analysis)--you've got some 'costs' of drinking fresh in your mind! Write them down on a little piece of paper and keep it handy. But focus for a few minutes on the 'benefits' part: not the benefits of quitting, but what you thought the benefits of drinking would be.
Were those benefits real?
Don't be afraid of your answer: we usually had reasons for drinking. If the benefits weren't real, describe clearly how they weren't and write it down. Remind yourself each day of how your perception was distorted or unrealistic. But if there were real benefits from drinking (it relaxed your mind, reduced your stress temporarily, etc.) acknowledge those benefits. Then start some goal planning: find ways to achieve those same benefits without alcohol.
CBT tool and concept: disputing irrational beliefs, and recognizing when a belief is irrational (unhealthy, maladaptive).
What are you going to do next time? Have your own mantra or slogan handy, and repeat it often. 'There will be no alcohol in my house or in my body' worked for me. What's yours? Do some role playing about future situations, and have in mind how you'll react, what you'll say, when you'll leave, what you'll do, who's going to help you. Be specific. 'I get a ginger ale immediately on arriving, and ___ and I leave after 45 minutes or as soon as one of us feels uncomfortable.'
You're not starting over. You just need to practice the tools for refusing alcohol more often: be persistent.
And don't hesitate to reach out for support. Too many people stay away from forums at the time they'd make the best use of them, because they're embarrassed to admit a mistake. Forums would be a pretty boring place if only the sober people posted.
Your experience can be helpful--far more helpful than you can imagine--to the others reading, the others too upset or afraid or embarrassed to step out ask for help. Your story is their story and our story.
Change is possible!