Yogaś citta-vritti-nirodhaḥ.

Patanjali, the sage who compiled the Yoga Sutras 2,000 years ago, knew exactly why. He didn't see your struggle as a lack of "willpower." He saw it as a structural issue of the human mind. He broke it down into three specific layers that act like gravity, pulling us back to our old selves: the Vrittis, the Kleshas, and the Vikshepas.