“I don’t” is experienced as a choice, so it feels empowering. It’s an affirmation of your determination and willpower. “I can’t” isn’t a choice. It’s a restriction, it’s being imposed upon you. So thinking “I can’t” undermines your sense of power and personal agency.

In other words, the phrase “I don’t” is a psychologically empowering way to say no, while the phrase “I can’t” is a psychologically draining way to say no.

https://jamesclear.com/eat-healthy