Occasionally I hear students say that they aren't very good at downward facing dog, or not flexible enough to touch their toes, or want to have stronger arms. This is usually in response to comparing oneself to others and coming up short. It is important to remember why we practice yoga. Yoga is not a competition, a skill to be improved on or a sport to be mastered. It is a practice that cultivates mindfulness, increases the flow of prana and calms the nervous system. Of course it also strengthens, stretches and purifies the body.
One of the social practices (the yamas) of yoga is "aparigraha", which means non-grasping or non-greediness. Usually this is interpreted to mean that we not take more than we need, or that we not get overly attached to physical possessions. However aparigarha can also refer to not grasping for personal qualities (I wish I was more patient) or abilities (I wish I was smarter). Grasping or wishing for something you don't have takes your awareness away from what is real - what you do have.
Next time you are practicing yoga, take it as an opportunity to practice aparigraha. Rather than wanting your body to be something different than it is, mindfully observe the body and breath without judgement or expectations. As you gain a deeper awareness of "what is", rather than what you wish for, you will witness your body/mind slowly and positively respond to your yoga practice.