Toppling out of a challenging Yoga posture is important, because our reaction is the true substance of the Practice.
The posture is just a petri dish to observe yourself in.
When you fall out, how do you respond?
Some of us get angry. Some of us feel defeated. There is frustration. Sometimes there is a feeling of despair or disgust. Ever get envious or competitive after flailing out of a posture?
Notice your reaction and then get back in the posture, but do so more attentively.
Be aware of what you can do differently each time; focus on a single, clear objective with every attempt.
The ability to perceive subtle information from your Body, and to command the Body with the Mind and Breath, is the greatest benefit of the physical Practice of Yoga.
Stay focused on listening more intently and being more present with every moment of your experience in-to, and out-of, postures.
With this mindset there is only feedback about the posture and yourself in a given moment. There is no such thing as failure when you are only exploring.
The only failure is to stop exploring.