If you are involved in a committed relationship, spend time with your partner in a romantic, pleasant setting. This will make you focus on the person you love instead of focusing on things that happened during the past and may happen during the future.
Instead of just thinking about how you can lower stress, write your thoughts out. Develop two lists for yourself: one that covers stresses you may have the ability to control and one that lists stressors out of your hands. Let go of the stresses that you can't control and concentrate on the list of stressors that you can change.