I can relate about the eating. I'll eat too fast and the message that the stomach is full hits after I've over eaten.
As far as one meal screwing up the day. Imagine opening the fridge and you reach for an egg. You accidentally drop it and it breaks. You don't have to drop the remaining eggs.
And if you screw up at one meal, one meal isn't going to make you gain weight. You'd have to eat 3500 calories over at that meal to gain a pound.
You got great motivation to get a handle on your fitness. You're going to be chasing that baby for the next 18 years or so....