Everyone has experienced different qualities of energy. Yet, at other times, you may also have great energy and be very focused and calm. Yoga involves learning to generate energy, and also to focus it into different parts of your body. This enables you to break through physical and psychological blocks, increasing energy, which allows new interest to come into your life. At any instant, the quality of your life is directly related to how interested you are in it.
Yoga involves far more than either having or developing flexibility.
Being able to do complicated postures doesn't necessarily mean you know how to do yoga. The essence of yoga is not attainments, but how awarely you work with your limits - wherever and whatever they may be. The important thing is not how far you get in any given pose, but how you approach the yogic process, which in turn is directly related to how your mind views yoga. Another one views the posture as a tool to explore and open the body. Whichever framework you're in greatly influences how you do each posture.