Who has tried a new challenge and not succeeded in that very instant? Many people, right? Think of learning a new sport - who hits a home run the first time at bat? What about basketball, as a new player getting nothing but the net on every shot is somewhat impossible. Then why do you think yoga is that much different?
Yoga is a skill. Over time you perfect the pose for your body, learn what your can and can't do and may adaptations. Keep it fun and still a challenge.
Always continue to challenge yourself and learn more!