If you have been practicing mindful meditation, you know that the essence of the practice is returning your attention to your breath when your mind wanders. No matter how many times it wanders in any given mediation session, your task is to return to your breath. Each time you do so you are building your mindfulness skills.
The same is true for mindful eating. Whenever you eat mindlessly, compulsively or obsessively (call it whatever you want), your task is to return to mindful eating with the next bite. No matter how many times this happens, start again. Bring your attention back to mindful eating the way you bring your attention back to your breath, and you will see that gradually, you will eat mindfully more often, without effort.