With increased and regular use, your body's tolerance level increases. Myself, I'll get a bottle (a fifth), drink about half one night half the when I drink next. I get drunk maybe once, sometimes twice a month.
You probably shouldn't doubt yourself. Learning your limits and being able to stay with them are a part of responsible use, especially if you are going to be a social drinker. My advice is to also stop before the world starts spinning. It's typical for pretty much anyone who drinks to eventually reach a "I'm tired of feeling like this" stage (I apologize if you are older than I'm guessing), so they learn their limits, learn to drink in moderation, and learn how achieve a good buzz without the bad effects of overdoing it.
As far as health goes it technically is best to never binge drink (but healthiest to have a serving or two a day a few times a week), but the occasional over indulgence isn't really that bad.
If I had to guess, I would say the newness of the experience is wearing off, and the realization of too much of a good thing is settling in. But most people who drink to excess reach this point, they don't like having a hang over, they realize they get stupid after drinking so much, so they set limits and stick to them. It's everyone's own personal decision, but myself, I promote the strength to say no or no more because you are strong enough to moderate yourself, rather than saying no out of obligation (except health, which as another issue) or a perceived weakness.
But, regardless, it's a sign of being mature and responsible with your drug use when can admit you don't like something about it, you can (and do) cut back usage, and are even mentioning things like limits and moderation.