Yoga is not about performing the sophisticated postures or to achieve the impossible. Yoga is for you to learn to unite your body, mind and soul. To enable you to know more about your body by understanding and observing how the body function and react while practicing the asana/postures and ACCEPT it.
The emotions of likes and dislikes, comfortable and uncomfortable, relaxing and suffering and many more will be projected while performing the asana/postures. Do not afraid of those emotions!! Without effected by the emotions and understand that emotion is just an emotion. How uncomfortable or suffering you feel, you understand the fact that it will pass. Practicing the detachment and let go the judgement and comparison of the asana/postures that you can and especially those you cannot perform. Soon, you will realise that all beings and things are impermanent. Even the feelings or emotions projected during the yoga practice are so real, but soon it will pass, it will pass.
Yoga is not about body flexibility, Yoga is about mind flexibility where you learn to accept what you can and cannot do. As simple as learning to accept yourself. That is how happiness walks into your life.
Happiness walks in not because of the impossible postures that you perform. Happiness walks in because you learn to accept who you are, you understand that things in life is impermanent and you started to practice the detachment, you learn to appreciate what you have and not being sad of you do not have. You learn to appreciate every moment and live at the present moment instead of the past or the future which does not exist.