You can't be genuinely happy if you don't believe it's possible. Pay close attention to your thoughts and words and how they impact your life. Perhaps you are repeating things you've heard in the past about being happy that keep you from experiencing true joy in the present. Let go of the messages that keep you from feeling balanced and fulfilled and replace them with words of encouragement and possibilities.
When you think and do positive things, you replace the thoughts and actions that were making you unhappy. People who live joyful lives understand that there will always be obstacles and challenges, but that they can get past them. To believe in happiness simply means that you are committed to following your inner voice and doing things that reflect who you are deep inside.