We spend our lives thinking about what we already did or are going to do, and we completely miss what we are doing.
Learn to stay in present. The past is over. Only now is real. Start observing what is happening around you. But you also have to analyze it objectively. Observe yourself in your daily life, every day. This mental exercise will strengthen you. Avoid free, spontaneous action as it is always blind reaction, which is always harmful. By learning to observe yourself, you will find that whenever a difficult situation arises in life, you can keep the balance of your mind. With that balance you can chose freely how to act.