I've had situations like this happen several times in the past when I relapsed with alcohol. I was on an anti-depressant and drank two beers at a bar and blacked out and didn't remember anything but waking up in my bed the next day not knowing how I got home. Based on my text messages from the day before I had been out drinking for at least 10 hours in a blackout. There have been more incidents like this too.
I'm an alcoholic and usually drink a lot more than that amount to blackout in the past. Then there have been times on an anti-depressant and Ativan (same type of drug as Xanax) that I drank more before blacking out. I guess what I'm getting at is even if you are able to drink more on the half-dose of Xanax, you really never know when this type of thing will happen again. It could be after 2 beers or after 12. Just cause you are managing it somewhat now doesn't mean it will not happen again. It's very unpredictable. It is really dangerous to drink on psych. meds. I've also found in the past that my behavior was different drinking a lot on psych meds than when not on psych meds. And not in a good way. Other people I know have had similar experiences drinking on psych meds. It is a really dangerous position you are putting yourself into. Make sure when you see the new therapist that you tell them about this incident and how much you drink. Otherwise there is not much point to the therapy. I hope you stay safe and get healthy.