20 beers a week could be the reason for your anxiety and headaches as well as sleeping problems/insomnia. You should not be drinking while on Xanax!!
Xanax is a benzodiazepine, which are known to cause a physical dependence, especially if used for 12 + weeks or on a dose over 4 mg a day.
I think you may be confusing "addiction" with "physical dependence". Addiction/Abuse is characterized by using a drug to obtain a euphoric feeling and knowing there may be a risk (health, law, etc.) but you take it anyways for the desired effect.
A physical dependence is characterized by physical "withdrawal" symptoms when the drug is suddenly taken out of your system.
Benzodiazepines can cause physical withdrawal symptoms such as muscle aches, muscle twitching, insomnia, appetite decrease, weight loss, problems concentrating, and anxiety among others. This is why your doc is tapering you off the drug (to lessen the effects of withdrawal)
0.5 mg a day is a very low dose, but if you've been on it for "years" then I'm sure you could have a physical dependence to it.
The beer will only make these symptoms worse (that is, of course, only after the initial happy intoxicated feeling goes away)
Personally, I would go get some Kava tea. Kava-Kava contains "kavalactones" which are well known to alleviate bad anxiety without causing side effects such as concentration problems or extreme sleepiness unless you drink too much of it. 3-5 cups a day could be very beneficial (if you can't stand the bitter taste, you could add lemon or whatever)- 6-8 cups may cause a person to have problems standing/walking because thier muscles are so relaxed. Kava's effects usually only last a couple hours and doesn't lead to a physical dependency, however it is such a (natural) relaxant that in 2001 it was banned in Canada, France, Germany, South Africa and other countries.
when you buy this stuff, BE SURE that it only contains the ROOT of the Kava plant (no stem or leaves!!)-DO NOT get Kava capsules (they don't work the same)- It's real easy to get high quality Kava tea in Hawaii and Fiji, or you can just order it online. - "Kanaka Kava" sells frozen and dried kava online. It tastes kinda bitter/ gross, but after one cup your tongue will probably be numb so it gets better.
You should quit the beer drinking soon. Since your a female, many beers will effect your hormones due to the yeast used to make the beer (Brewers Yeast effects female hormones differently then it effects males) The hormonal changes could lead to weight gain, moodiness-anxiety-etc. These hormonal changes are so severe that if often leads to an increace in female breast size. - which may or may not be a good thing, I'm just trying to point out that beer screws up the female body more then it screws up the male body.