How to stop eating after being full?

I have quite the problem with food. I'm like, the garbage can of the family, haha. I'm not sure if grandma doesn't know she's making too much food, or the rest is just pickier/don't eat so much anymore, but we always have leftovers for like... 3 to 4 days? Which wouldn't be bad if she wasn't border most days, which ends up with her doing something fresh each day. And someone have to eat leftovers, or it will spoil.

I know it sounds like an excuse, but I really don't know how to stop? I caught myself finishing like, bag of salted peanuts after because it was opened for a week and was just sitting on the table. Which like, it wouldn't get spoiled! But I still ate it anyway even if I was supposed to be full. I don't think I can actually feel hungry. Most of the time I want to eat something (sweets or snacks, because dopamine), but when it comes to breakfast, dinner, it's just because I have to. And then I have to finish the plate, because it will go to waste. And then I have to finish the salad no one will eat anymore because it will spoil till the tomorrow.

I just don't know how to stop. I feel like it's ingrained in me at this point. Getting on any diet never worked because I'm still living there, and then grandma still makes so much, and then someone has to eat it. And the worst part? Most of the time it shouldn't even be a big deal, we have cats, dog, chickens, and a composter, so throwing there the spoiled food won't be wasting it, and yet I still can't let go.

Is there a way to be more assertive? Not to eat when not actually hungry?