Focus on what you do want. Make your goal explicit. "My goal is to increase my moments of joy." This way, every single moment of joy is a successful moment.
Celebrate each moment of joy. Be grateful every time you experience joy.
Having this goal will place your attention on joy. Instead of feeling bad when you are not joyful, you will experience positive feelings about experiencing more joy.
Each moment of joy in your entire life is experienced one moment at a time. You can't have more than one moment of joy in any given moment, but you can increase the number of joyful moments. How? By focusing on it.