Practicing this ashtanga yoga is a funny thing. On the one hand, doing the same poses every day is a good measure of progress over time. On the other hand, those same poses that were once easy can be difficult and vice versa and that can do a number on your mind.
So, for me, part of the practice is letting go of what you can and can't do (not meaning that the physical effort to practice isn't put in) and taking each practice for what it is. Some days things come easier, some days not so much. And it's all good.
How well a person does a pose does not reflect how "good" a yogi she is. Being present in the moment, breathing, and letting go is what it is about.