In his 1988 book Becoming an EX: The Process of Role Exit, the sociologist H. Ebaugh talks about the process of letting go of a particular identity. What you need to do to become an EX-smoker, or EX-overeater, or even an EX-wife, is to reestablish an identity separate from the prior identity.
Maybe you have lost weight in the past, only to find yourself gaining weight once again. Perhaps you were not able to let go of yourself as that "overeater". You lost weight, but maybe you weren't so comfortable - maybe you felt something was missing. As long as your new found weight loss makes you feel like something is missing, you will be longing (secretly or otherwise) to return to your old behavior. Therefore the danger of falling back into your old ways is much too real.
Successfully staying a thinner person requires that you actually change your perception of yourself. You need to really inhabit your new body, heart and soul. You need to build a new healthier relationship to food, and a new sense of being whole and complete without overeating.
This doesn't need to happen all at once, but you can be working on it. Your aim is to become an EX-overeater, or an EX-unhealthy eater, or an EX-compulsive eater. That is how to safeguard your hard-won weight loss.