Focus on your attention
Mindfulness trains you to contemplate your thoughts and feelings without diving into them and without judgement. While practising, you learn to be attentive in a curious and gentle manner to both external stimuli (such as sounds and smells) and internal ones (like thoughts, emotions and bodily sensations).
While practising mindfulness you focus on your breathing. Every time your thoughts wander off, you redirect your attention to its anchor point, your breathing. In this way, you start to perceive nagging worries, and you learn to simply be aware of them and observe them, and to distance yourself from those negative automatic thoughts.