It's not the pose. The hardest part about practicing yoga is not putting yourself into a challenging pose.
The hardest part about yoga is balancing the opposing pairs. Balancing the opposites. Bringing one-pointedness to the duality of life.
Nowhere is this more evident than in the statement "Abhayasa Vairagya." This statement can be found in the Yoga Sutras of Patanajali (1:12).
Abhayasa can be translated as - Constant Practice. Diligent practice. Determined practice.
Vairagya can be translated as - Complete detachment of the results. Non-attachment. Letting go.
Thus, the hardest part of practicing yoga is PRACTICING diligently without thought of an "END GAME."
One has to practice with determination - "Never give up" - but - "Always letting go."
This concept is incredibly hard to grasp for most people - let alone practice. Why would we want to practice so hard - dedicate so much work - if it wasn't to ACHIEVE something of value.
The practice of yoga is about doing the work, without thought of result. Just do the work. Do the practice. And the practice is to let go. Then you have mastered duality and nothing externally will move you.
Namaste - Jim