For myself and others, depression manifests itself as ADHD like symptoms. Something anxious takes up residence in my mind, and until it’s resolved, it can dominate all other thoughts. Combined with a million distractions, and I seem to loose all ability to focus on what I want. But in reality I am just focusing, or avoiding focussing, on something else.
We are not machines. Concepts like willpower lead people to believe they can just choose to focus on what they want, when they want without concern for what’s going on in their lives.
Mindfulness is a step in the right direction, but it is not a panacea alone. You still have to address what you discover through self reflection.