"Imagine two people resisting a cigarette. When offered a smoke, the first person says, "No thanks. I'm trying to quit." It sounds like a reasonable response, but this person still believes they are a smoker who is trying to be something else. They are hoping their behavior will change while carrying around the same beliefs.
The second person declines by saying, "No thanks. I'm not a smoker." It's a small difference, but this statement signals a shift in identity. Smoking was part of their former life, not their current one. They no longer identify as someone who smokes.
Most people don't even consider identity change when they set out to improve. They just think, "I want to be skinny (outcome) and if l stick to this diet, then I'll be skinny (process)." They set goals and determine the actions they should take to achieve those goals without considering the beliefs that drive their actions. They never shift the way they look at themselves, and they don't realize that their old identity can sabotage their new plans for change." - Atomic Habits
Ofcourse this skill is not straightforward as it seems. You will see that it doesn't always work. The reason is the same as everything else, you have to ease into it. If you're 100kg and you start saying "I am a healthy person" you may have congnitive dissonance. So it might make more sense to tell yourself, "I am someone who acts like a 90kg person" once you hit that mark you can go down to 80 and so on.