If I had a penny for every time I started and stopped a diet, I would be a very rich woman right now.
Eating yummy convenience foods and holding down the couch is easy. That's why so many of us fall back into familiar (and unhealthy routines). Prepping veggies, journaling everything you eat and saying "no" to people when they tempt you with stuff you know you shouldn't have is *hard*.
I work in a hospital and part of my job is to encourage smokers to quit. They tell me that as a non-smoker I don't understand how hard it is to stop doing it. I don't know what its like to smoke, but I DO know what its like to not be able to stop stuffing food into my mouth, even though I know its bad, and I'm full and I just shouldn't. Its a compulsion a lot of the time. Maybe I don't do well 100% of time time, but if I can eat a healthy variety of foods in a reasonable quantity even 50% of the time, then I'm doing better than I was. Eventually I'll eat well more often than not.
Take it all one day at a time. Don't let slip ups derail you completely.