Mindfulness is living in the present moment. It's paying total attention to what you're doing now, instead of being anxious about the future, or worrying about the past. It's an incredibly powerful way of taking your mind off problems, real or imagined.
If you're constantly living in the past or future then you're not paying attention to what you need to do now to get to the future you want. What happens then is that you get one you don't want as you were too distracted to attend to the things you needed to be doing when you had the chance. It's never too late to change from the mode that most people operate in and one where you get in the flow of the present. Take care of the now and the future will take care of itself.
Here's a little exercise you can do for yourself. Get into a comfortable position. Close eyes, breathe in & out, paying attention to your breath, how it feels when it goes in & when it goes out. We always have our breath with us unless we’re dead so it makes a great place to focus our attention when we need to. As you sit there breathing in, breathing out, you neither need to slow down your breath, or speed it up, you can pay attention to those sensations, in this experience, in this moment, without regret from the past and without worry about the future. If you find yourself thinking about how you’re doing, or about anything else, just return your attention to the breath. Paying attention to the chair below you, the feeling in various part of the body, noting what these things feel like in the here & now.