If you are constantly dwelling on the past or daydreaming about the future, you are missing out on the present moment. Remember that the past and the future are simply illusions, and that real life takes place here and now.
Start paying attention to negative thoughts so that you can learn to silence them and enjoy the present moment. If a negative thought arises in your head, then acknowledge it, label it a negative thought, and then let it fade away.
Get in the habit of paying attention to the small details around you. Appreciate the feeling of the sun on your skin, the sensation of your feet walking on the ground, or the artwork in the restaurant you are eating in. Noticing things like these will help you silence a rambling mind and appreciate every moment.