I don’t want to forget about the second part of your question, “why is my brain (me!) being so negative with its prediction of what I’ll do later, rather than letting me be pleased at my success?”
I agree with you that this aspect is fascinating, and the way I understand it is that “you” are something other than your brain. At least to a very large extent (and maybe completely) the brain replays what happened in the past, automatically.
My guess is your brain says you will overeat later anyway simply because you’ve done that before. And it’s always good to keep your options open: maybe you’ll overeat later and maybe you won’t.