Do you ever look at an advanced yoga practitioner in an awe-inspiring pose and think to yourself, "I would never be able to do that!"?
Join the club. Most yoga students can relate to being envious overwhelmed, or discouraged when they see someone doing what seems to be an unobtainable pose. But instead of concentrating on your own shortcomings, next time try to focus on the potential in your yoga practice—if not physical, then certainly spiritual.
Although there will always be exceptions, most accomplished yogis didn't come out of the womb with the abilities they display in those awesome postures. They used yoga as a tool to gradually improve their flexibility, strength, and concentration—just like you're doing. And remember, yoga isn't about being able to shape your body into a pretzel. It's about what you learn about yourself while you practice that matters.