Even on your most challenging days, you deserve to feel happy, even if only for a moment. Cultivate happiness in your life by doing something each day that makes you smile, so that on the hard days you have plenty to pull from.
Recall a fond memory. Engage your senses. Dance like a total weirdo. Laugh at a terrible joke. Play with your pets/kids/best friends.
Whatever makes you feel really happy, allow yourself to experience that feeling every single day.