In conversations when I mention that I'm a yoga teacher people often respond "Oh, I can't do yoga, I'm not flexible." Or occasionally I get a new student with some experience who classifies herself as "bad at yoga." Both of these statements are completely untrue and should not be uttered by anyone.
No one practices yoga because they are flexible but rather to become flexible. Or for those rare folks who are bendy, to become more balanced in their flexibility. Besides, flexibility is only one component of yoga, there's also strength, balance, breathing, relaxation and concentration.
More importantly, yoga isn't about being good at it but rather doing something good for your body and mind. I like to remind my students that yoga is journey and not a destination. You'll never get to the point where you can say, "Yep, I've mastered yoga, I don't need to practice anymore." Yoga is ongoing because our bodies are constantly changing. As we age our needs change too. Younger people may need yoga to stay in shape and unwind while older folks may need yoga to help deal with everyday aches and pains. You won't be doing the same poses at 60 that you did when you were 20 or at least not in the same way.
I find it a bit troubling when students label themselves as "bad" because it means that the judgments that plague them in life have followed them onto the yoga mat.Your yoga mat should be at least one place where you can let those judgments go and simply be as you are. I like to ask my students at the beginning of class to let go of their ambitions toward their poses. I think when you set the tone of your practice this way it is much more enjoyable and frees you from expectations. So much of life is performance based why not make your yoga practice one place where it's not?
Besides the mental frustration and stress judgments create, they can also lead to injury. When you want to be good at a pose or try to force yourself into a pose you risk hurting yourself. Listen to your body, it will tell you when it's ready for something more challenging. I like what B.K.S. Iyengar says: "Challenge yourself, don't abuse yourself." One minute of glory isn't worth the three months recovery if you attempt a pose that you aren't ready for. Accept where you are in your practice and be content with it.