I understand your feeling, Tdol. It didn't work for me either, at first. But we must allow that context is a mighty important factor in determining the efficacy of any given collocation.
In addition to obstinacy, which I'm not saying is the case in the subject sentence, we also have the notion of regret. An 'only' isn't absolutely necessary to effect that feeling. Intonational shifts can also illustrate the 'only' feeling.
Iiiiif IIIII would eat better, IIIIII would be healthier.
Something that is difficult to show in writing.
