Overeating can be a major problem even for healthy people, but the solution is more complicated than just putting your fork down. Your willpower is limited, and the mechanisms that make us overeat are hardwired, as this video explains. All is not lost, however.
This video from ASAP Science explains the science behind why we overeat -- namely that when your stomach is empty, Ghrelin, a hunger-stimulating hormone, is released and tells your brain to eat. Normally, when you're full, your body releases another hormone called Leptin, which kills your appetite. It's not that this mechanism is broken in modern humans: The theory is that high energy (fatty, sugary, carb-loaded) foods used to be scarce and our bodies are biologically wired to crave them when they're available, overwhelming our body's natural hungry/full response. Since we can get them all the time now, we want them all the time.
So how do we fight back? Since your willpower is limited, bolster it by ditching the junk and rebooting your diet. Surrounding yourself with the good stuff will ensure you eat the good stuff. Also, take it slow and make smart, sensible changes that make it easy to stick to your plan. You're building a new, healthy you, not "dieting".