You have two kind of unrelated questions here. Food turning into energy is a chemical process where the sugars/nutrients are broken down and transferred to the rest of your cells. But, that has really little or nothing to do with feeling full.
Feeling full has a variety of physical and psychological causes:
This is also why most unhealthy foods are unhealthy. They pack a lot of calories into a small size and thus we can eat a lot of them before our stomach fills up. Soda is one of the worst things you can drink because it is a liquid that barely fills you up physically but delivers enormous amounts of sugar. But anyway, another reason is
You see how much food you've eaten. That sounds silly but studies have shown that people take longer to feel full if their plates are cleared away fast so that they can't see how much they've had. This is a good reason to cook your own meals and clean up after - not only is it a great skill, but your brain develops a better understanding of how much you've eaten and you're less likely to overeat (whereas if you go to a restaurant or fast food place you really have no idea what you're eating and you just throw out everything afterwards, losing all that information).
Hormone signals are also released during digestion, and most of these signals go to the "satiety center" of your brain, which tells you you're full. Animals that have damage to this area will keep eating and eating because they never feel full. Possibly, individual differences in humans might be due to this area being over- or under-developed.