Pricey, I was talking about this with somebody here yesterday. I'm of the opinion that if you have to lie so that you avoid embarrassment and hassle, then do it. The more I say this, the more people that recognize my screenname are going to think I am a big fat liar, but I can't help it: I can't think of anything smarter to say! I compare it to another sensitive topic that is not worth exposing in all its nudity in front of others. Like giving "too much information" or casting a black cloud over a situation by mentioning something that sounds sad. It doesn't sound like "lying" when we keep such things to ourselves in that light; it's just decorum. Not everyone is ready to hear the full truth, let alone capable. So: I would construct the best response. People usually suggest using "my doctor doesn't want me to drink while I am X" or "it hurts my stomach" or "I haven't been able to ever since I got sick that time I had food poisoning" - something like that.
People also suggest avoiding the situations, but there is only so much of that you can avoid or should avoid. All depends on what your recipe for healthy living is.
I think you should remember that it is going to be a process, that you can't solve everything in one minute. At least this is what I have told myself, and I have often wished that I could. I think it's a form of perfectionism, if I understand what that means in the first place. I avoid the temptation to picture huge mountains to climb. It's taking a little getting used to, because I like looking at the big picture, or at least believing I know what it looks like. Sometimes we should accept that things are taken in small steps instead and learn to relish those. And we build (hopefully) one block of confidence on top of the other. Anyway, if that makes sense, maybe that will help you with the building anxiety you have noticed when thinking about upcoming social contexts and the whole "How come you're not drinking?" malarkey.