I always thought of mindfulness as focusing on one's thoughts & feelings then acknowledging and accepting them. Like if you feel good, you're aware of it and you focus on it. If you feel bad for whatever reason you accept how you're feeling in this moment is what it is, you become aware of it and don't fight it, what you resist persists after all. If your current reality is bringing you down you realign with better thoughts when you're in a good place. I hope this makes sense because it sounded a lot clearer in my head. 

I've got to say after really practicing it I have been able to let go a lot easier. It helps ground me in the moment and from there I create the thoughts I want to manifest. Sprinkle some feel good heart energy and release them. If I ever feel like I'm about to slip into doubt I just re center and focus on the act of breathing.