I know with Evan, the more I want him to do something, the more he enjoys not doing it. As for yelling, I've had more episodes of that than I like to admit. It was really bad between 18-20 months, I think, but now I hardly ever get that frustrated at him, even though he's definitely moved into the terrible twos.

I know perfectly well that yelling doesn't help, but he pushes my buttons and I'm off to the races. At least your frustration is born from a noble cause: you're worried about her weight and health. My frustration is born only from my unreasonable expectation that he act like a rational being.