Hi NewLeaf,
I'm facing this too. My partner knows the full story, but most other people in my life don't know and I've been thinking about how I will deal with this over the festive season (I've avoided most drinking-based social events over the past 2 months - not too difficult, as I've been very busy with work).
I don't know if this will work for you, but I'm planning to be as confident and upbeat about it as I can - i.e. not at all apologetic. I'm going to say something like "I just don't fancy it anymore - I've started having a really bad reaction to it. It's not doing anything for me". I'm hoping that if I'm very matter of fact about it, people won't make it a big deal.
I've also found it helpful to notice how many people in bars, restaurants etc are on soft drinks. I always feel like EVERYONE else is drinking - but if you look around, you'll see that's not the case. You won't be the only one.