The willpower to say NO

I always feel like I have a mental fight going on in my head everyday.  I look at what is on my plate (or someone else's plate for that matter) and think should I put it in my mouth.  Yes or No?  Then I try to justify why I should eat it and I'll eat less at my next meal or plan to walk it off.  Why can't I just say no?  Why?  I know what I need to do but sometimes I just don't do it.  Wished my No's would always win against my Yes.