Right, it involves both aspects, the quantity and the psychological addiction. Of course if you drink much every day you're an alcoholic, wether you know it or not.
If you drink a beer and feel edgy if you miss it then it's certainly not a real addiction to the substance, but a psychological problem (it's almost as you're addicted because you think you are, even if you actually aren't). Of course it's still less dangerous anyway, better being addicted to a beer after work than to 10 cigarettes a day (which is another psychological problem, it's true cigarettes contain addictive substances, but if you don't smoke too much you can quit when you want, I always do when I have flu, I don't feel bad and then I start smoking again only because I need it with my mind who likes the taste and to release tension).
In this case, I just wouldn't worry. Don't feel guilty for a beer, but don't feel bad if you miss it. Take it easy.
Then try to miss it for a couple of days, but don't wonder "oh what will happen when I get home and don't dtrink my beer", don't worry, it won't happen anything unless you expect it to happen. Sometimes we build ourselves our problems. If you don't worry it'll be alright.