The work of mindfulness is not to defeat a distracted mind, or control thoughts. It's to calmly interrupt distracted thoughts, feelings, and behaviour over and over again.
Then choose a focus on purpose, over and over again. Eventually your brain stops being used to distraction, and finds more clarity and contentment in the pauses between things.
Check out this article from Bhante Gunaratana on some practices to work with distraction.
From Mindfulness in Plain English by Bhante Henepola Gunaratana (Wisdom Publications, 1991)