I don't doubt many people may need help with nutrition education. It's remarkable just how much garbage is in things packaged as food, and remarkable how even the quoted figures of nutritional content are often largely meaningless because the idea of what counts as a serving can be so strange. I honestly have no idea how many different products, usually dessert-type products, I've seen where I'd look at the packet and figure it's probably two servings only to look at the nutritional panel and and see that the numbers are based on it being 4.5 servings. So if I planned to cut the thing in half and share it with my wife, I need to figure we're looking at slightly more than double the quoted figures per actual serving on the plate.
It's pretty sad that the food industry is allowed to use figures that look like they are intended to mislead, and no less sad that so many people can go through the educational system without learning about the basics of eating healthily, balancing healthy food with a suitable level of indulgent food and so on.
Looking at the calorific content of candy bars in the context of taking some exercise can be eye-opening. I weigh around 215 these days and during my exercise yesterday I walked a little over 5.5 miles. My tracker estimates I burned a bit over 600 calories in the process. A large candy bar can easily contain 4-500 calories. When I think that the effort I put into that walk can be undone within the minute or so it takes to eat a large candy bar it really makes me not want the candy bar any more. But that also requires education, and a reliable estimate of actual calorie burn. A while ago an overweight friend started exercising and posted a lot of his workouts, along with estimated calorie burns on social media. His estimated calorie burn figures were many times higher than anything that was realistic, but they were the figures a cheap exercise machine spat out. Had he been using them as a guide for eating he could have been undoing all his work while thinking he was ahead. If his machine said he burned 800 calories and he figured he could afford to have a 300 calorie treat, but he'd actually only burned 200 calories, he'd end up taking on more than he burned off. (I knew his figures were far too high as I've done similar exercises with a power meter so I could accurately measure energy usage)
Where health is concerned I think we get far too concerned with the idea that something can be hurtful. When I was at my heaviest I was fat, there's no other way of putting it. My BMI was only a whisker under the level to be considered morbidly obese. Eating less and moving more isn't a rapid-fire way to lose a lot of weight fast, but a sustained lifestyle change makes a lot of difference. The crucial thing that has to happen is that the overweight person needs to want to commit to making a change and accepting that there are no "life hacks" to shed a pile of weight in a week. It took a while before I got to that point but once I did things started to change. For good measure being outdoors, even if only exercising very gently, brings a whole host of other benefits.
Another problem with the educational aspect of everything is the idea that everything needs to be done at once. You mentioned people failing on strict diets, so part of the point of a diet is that it needs to be sustainable. If someone decides that as of tomorrow they eat no more candy, no more chocolate, no more soda, no more chips, the result is that they will crave those things. Their body has probably got so used to the steady flow of trash that it wants more and a diet that is totally unyielding will prove unsustainable, at which point many people will figure it's not possible and give up. The diet needs to be based on cutting back rather than cutting out. Sure, you can have the candy bar or the bag of chips, but pick one rather than having two of each. Slowly fade things out of the diet as you figure you really don't need them. Swap things for other things that are also enjoyable - when I cut out soda and switched to unsweetened iced tea when eating out the result was remarkable. I shudder to think how much sugar, and how many dead calories, I was drinking.
