Changing your thoughts changes everything.
The nature of our thoughts determines the quality of our life. This is not an overstatement. Your thoughts will determine whether you are content or discontented, optimistic or depressed. Many times, you will find that simply by having an "attitude of gratitude" and changing your thoughts to focus on your many blessings, will change your entire feeling about your life -- even though the circumstances in your life haven't changed at all.
What did change? The way you chose to think about your life.