In most situations where we find ourselves saying: "Just one more," we'd be better off if we quit before then. Yes, there are those who motivate themselves to do one additional push-up, to run that extra lap or to finish another task, but mostly "just one more" becomes a justification to extend our pleasure to a point of excess.
"One more" usually means reading another social media post instead of sleeping, having another cookie although we have already had two, putting down another bet at the table when we have broken even, ordering an additional drink after we've had plenty or playing another round of computer games rather than being productive. When we gain pleasure from doing these things, the hit of dopamine comes and overrules the rational logic telling us to stop.
"Just one more" is a curse -- a sure sign that we have set meaningful boundaries that are about to be violated. The short-term impact can be minimal, thus rationalized away, but the cumulative effect of ignoring parameters can have serious negative consequences. One bet, one drink, one pound, one hour later and the momentum takes us into undesirable territory.
Create your limits before the temptation is knocking, and then stick to them. If you hear that voice in your head urging you to give "just a little," know that it really is a disguise for giving up a lot.