I used to drink like a madman (developed into full on alcoholism) - as in my usual consumption when I *wasn't* going out was a bottle of red wine and a half-bottle of sambucca; or an entire bottle of whiskey. 1st year of uni and everything - led to me dropping out.,
These days, after stopping drinking for a year or two, a couple of pints manages to get me drunk (working on it - it's embarrassing now I'm a student again). I physically can't drink more than 4 pints, even on an all-nighter.
Can still drink spirits though, which in all honestly I prefer when clubbing (as a couple of pints is sufficient when sitting in the pub with mates). However I don't go out often, mainly due to uni work building up and a general dislike of Birmingham nightlife (I miss Nottingham and the Goth club in Southampton).
Either just accept that you're going to drink less (getting hammered is not necessary - speaking as someone who used to do it often, it's bloody stupid really). If you see a Doctor they're just going to tell you to lay off the booze anyway. All that's probably happened is you damaged your body a little drinking so much at new year, and you psychologically/physiologically can't handle as much booze any more - it's not life-threatening (as you'd know by now if it was proper poisoning or anything) it'll just mean you live a little healthier!