For years and years, a decade or more, I was like that. I drank a bottle of wine every night. Just one. I kinda knew that was more than most people drank but I also didn't feel it was excessive. I went to work, I paid my mortgage and my bills, I didn't have hangovers really. Then I would stop for a glass of wine at the pub on the way home. Just one. Occasionally two but mostly just one. I'd have my usual bottle at home and not notice that with two at the pub a bottle at home, well that's a bottle and a half. Somewhere along the line I started needing to make sure there was two bottles at home for the night and I'd leave half a one for later. Later being 3am when I woke with terrible insomnia and couldn't get back to sleep. Ah, that's what that the rest of the second bottle is for. Up at 7am, working still, still no real hangovers but getting groggier. Fast forward a few years, and it escalated pretty fast after that. At the end of my drinking I was drinking around the clock. First thing in the morning, day times, until I went to sleep, in the middle of the night. During all this, I still ran my business, paid my bills and to the outside world seemed functional. I didn't think my life was unmanageable, but in retrospect, of course it was.
Someone wise on this site said that 'functioning' is not a type of alcoholic but rather a stage of alcoholism. I would agree with that.