Not paying attention means you have focused internally, retreating from the world.
Living in your head means you miss the world and enriching details. That is, you miss out on life for a fantasy.
Of course, the external world may be a nightmare, and fantasy is your reprieve, or perhaps your internal ruminations are terrible, and you avoid your mind to survive. But the only way out of regretting a life of escapism is to begin anew in the present.