you have to break the cycle of immediate gratification. if you see candy/pizza/donuts, you have to teach yourself that this isn't your last opportunity to have them, despite what their advertising campaigns will tell you. and skipping them this time doesn't mean that you aren't ever going to be able to have them again. those sorts of things are ubiquitous. they pretty much will always show up at certain places, and even if they were one day to cease magically appearing through no effort of your own, they're pretty easy to search out and get. and yes, this took a long time to sink in.
don't start trying to work in the candy today. plan to work in the candy tomorrow. the more often you can delay the gratification, the more likely you are to be distracted by something else and forget about it. because once you get to the point where you can want candy and have it tomorrow, you can work on wanting it now and delaying having it til two days from now. and then move that out to three days, then four and then five. this way you can slowly teach yourself that having candy doesn't mean you have to have it right now. and not having candy right now isn't suffering.
and if you can't do something for longer than a week, you need to start where you are rather than where you want to be. in other words, instead of starting at your ideal diet, start where you are, off the wagon. look at those foods that you make and consider staples and figure out small changes that you can make that make those choices that you fall back on better for you. so if you love mac and cheese, have mac and cheese. but see if you can cut out a little of the oil or fat in it and see if you can add a serving or two of veggies to it. the last time i made mac, i used half pasta and half zucchini cut up to be the same shape as the pasta along with some milk, grated in cheese and spices. i didn't immediately start using zucchini in my mac, i started by adding maybe 1/4 cup of zucchini to the total amount of pasta i was using. once i had been making it that way for a while, i cut back on a little of the pasta and doubled up on the zucchini. after i got used to that, then i cut the pasta back again and added more zucchini.
and when i say get used to it, i'm not talking making it once or twice and calling it good. i am talking about making it the new way often enough that you start to think of the new way as simply how you make it. in other words, that you do it in the new manner to the point where you're automatically doing it the new way without thinking about it. which can take weeks and a lot of repetition. because it can take six weeks to change just one habit, and everything about the way that you eat is part habit.
-google first. ask questions later.