Getting caught up in "should" is a killer of all forward momentum, in life and on your mat. When you stop resisting your reality and instead choose to embrace it, you can really shift. Instead of "My Crow sucked—I was only up for a second," try thinking, "My Crow got some air!"
We don't just practice yoga, we engage in sensitivity training—with ourselves. No one yoga practice is the same; it's as unique to you as your fingerprints. And nothing—not the voice in your head, your teacher, the person next to you, or yesterday's practice—should dictate today's. Instead your best yoga practice will from focusing always on easefulness, challenge, and integrity.