Every time I apply it consistently it works consistently.

And yet, as you note yourself, you live in a world where you've repeatedly been unable to maintain your dietary changes for a variety of reasons. So "you can lose weight by reducing your calories" may well be a true statement but is clearly not sufficient when we're thinking about public health interventions and messaging. You can teach people this fact but what else is needed to allow them to action it in a way that is maintainable for the rest of their lives?

The problem with “diet” language is that infers a temporary program. The successful strategy for weight loss is a lifestyle change mindset.

Broadly yes. "Going on a diet" really needs to be shifted to perspectives around dietary change. Of course that change has already long been changed among medics and dieticians but popular discourse lags behind this by quite a margin

What’s the alternative you’re proposing?

I think it is an important part of this as a public health issue that isn't as well studied so far so I'm not sure I (or anyone) can give you a reliable good advice here. Firstly the advice to exercise to lose weight should be struck from the record, that's consistently shown not to ever work in real world settings for the aforementioned reasons. Reducing calories does at least have real world impacts, albeit temporary for most people, so that can stay but as I say needs substantial supplementation.

Wider policy that actually enables people to make lasting dietary change I suspect is quite harder and likely beyond statements that apply to individual. I suspect people who lead less stressful lives can maintain changes that require willpower better. But reducing people's stress levels requires things like better paid work, shorter hours and more free time, better support for families, more affordable healthy foods, healthier ready meals, elimination of food deserts, etc... But these are systemic issues that require other political interventions so I don't really hold out much hope for any of that being addressed.

I suspect some kind of community support networks could work, like weight watchers but with an actual evidence base around what does or doesn't work with that approach. And we need better knowledge about what happens when people leave such programmes so they can be supported to maintain what changes they've made.