Just like with quiting anything (like smoking), you have to WANT to do it.
ITA! It also takes a while to adapt your taste buds and to ween yourself from the artificial stimulation many 'edible substances' (should we even call some of them food?) provide.
I used to be a junk-food junkie through adolescence and high-school. When I went to college and got involved with a food co-op and started reading about nutrition and health , I not only became a near-vegan vegetarian (then vegan), but changed to eating primarily whole foods (some people may call that 'clean eating' now, but I don't like that term) . Easy to do back in the mid-70's, when not a lot of processed vegetarian/vegan food was available.
I've had ups and downs from there, and sometimes get back into eating junky food (especially since it's so easy to get vegan junk food nowdays!), but some things I used to consume all the time (like Pepsi or American white bread) I find positively repulsive now.
I sometimes think of how the teenage me would view how I eat now, and she would probably find some of the things I eat (green smoothies? whole-grain sprouted bread? quin-what?) as yucky as I find what she used to eat!
I find that educating myself, and reading about health benefits of whole foods (Dr. Fuhrman, Brendan Brazier, raw-food authors) helps keep me more on track.
Also, if you can think of food as fuel, and that everything you consume becomes part of you and goes to build your future body, wouldn't you rather fuel yourself with healthful, good things than with something that offers perhaps momentary pleasure (yum!) but may actually have a negative affect on you? Take care of yourself the way you'd take care of an expensive sports car: give it the best fuel. Not like you'd treat a junker you don't care about, giving it the cheapest fuel possible.