Do you know that sometimes in order to master a challenging asana you have to stop practicing it? As counterintuitive as that seems, it is true that sometimes the best way to practice something is to not practice it at all. Actually, to say "not practice it at all" isn't entirely accurate. It isn't the case that practice stops outright; instead you shift the focus thereby making it a kind of indirect practice which, in some cases, turns out to be better than the more direct route.
We already know all about "practice"; we have discussed it at length -- what it is and what it isn't, how best to establish as well as to sustain it, and that it is both for our body and for that something which is more than just our body. We also know that, according to Patanjali, true practice emerges from efforts which are constant, consistent, and passionate (see previous post: "What Does Dirgha Kala Mean?").
But that is only half of what Patanjali says is necessary on our part if we want to experience the kind of stilled consciousness he thinks is yoga's ultimate goal. If you want the physical and mental peaceful well-being that eliminates suffering, cultivates authenticity, and bolsters happiness, it isn't enough to practice, even if you do it really well. You must also be nonattached to the outcomes of your practice.
The word for "nonattachment" in Sanskrit is vairagya. It also means non-reaction, indifference, and disinclination. Its roots mean dislike and even hatred. Yoga tends to refrain from hating things, of course. And this isn't about liking or disliking. Patanjali is encouraging something less extreme here; namely, an attitude of neutrality.
We are beings easily moved by sense perceptions. That is to say, whatever we see or hear or touch creates desires in us. And we are naturally inclined to satisfy our desires: when we desire something, we are tempted to act in whatever way alleviates the desire. What is important to understand is that it isn't the wanting which is problematic. Lots of wants are good and necessary. When you are hungry, you want food. When you are tired, you want to sleep. There is nothing wrong with satisfying those desires assuming they are genuine and not manufactured for the wrong reasons.
What is problematic is when you act upon the desire with the intention of merely enjoying or benefiting from the outcome or result. If you eat, for instance, in order to relieve boredom or just to experience the flavor in your mouth without being truly hungry, that is an attachment that Patanjali says impedes the yogic path.
Vairagya is about balancing discrimination and contentment. First you must know the difference between wants and needs. Then you have to enact willpower in order to act reasonably toward those things. Discrimination in that sense is a kind of heightened self-awareness, and being skillfully particular allows you to make good choices. When you make good choices you are less likely to be tossed around by feelings (either positive or negative) associated with those choices. That is to say, when you have made what you believe to be a good choice, the results, for better or worse, are easier to accept. And contentment and acceptance are keys to nonattachment.
Remember practice is about affecting change; it is about producing outcomes (see previous pose: "First the Fish"). Nonattachment is about being genuinely okay with those outcomes regardless of their states or characteristics. Practice is about wanting something. Nonattachment is about understanding why you want it, and knowing how to achieve or receive it properly.
There are as many different ways to cultivate nonattachment as there are ways to practice. In fact, nonattachment is itself a kind of practice requiring constant, consistent, passionate effort. You can plant the seeds of nonattachment while on your sticky mat by watching your reactions to the way your body performs asana. Choosing to practice poses which are supportive and empowering with an attitude of curiosity and experimentation while abiding in patience and satisfaction is vairagya.
Of course that is all easier said than done. And the stronger the desire, the trickier it is to uphold. The more you want to perform a pose and the more strongly you desire mastery over it, the more difficult it can be to let go of your attachment to that achievement. And that is where cessation of its practice can prove the most useful.
Sometimes in order to master a challenging asana you have to stop practicing it. By that what I mean is practice something else. When some given pose is persistently challenging and unachievable, it probably means you are not ready for it. Some physical or mental or energetic necessity hasn't been developed in you yet. And continuing to plead for it and chase it will only stoke your ego, agitate your mind, and preserve restlessness. Not to mention the fact that it can put you at risk for injury.
Instead, if you shift your attention to some other (directly or indirectly related) area(s) of practice, you can continue working toward strength, mobility, patience, resilience, graciousness, awareness, and all the other things that arise from good practice. If you need longer hamstrings or stronger arms or more humility or less criticism, you can create and develop those things in other poses and in other experiences.
What often happens if that when you do return to that challenging asana weeks or months or even years later, it's not so challenging anymore. You have prepared yourself for it, you have made yourself ready. You are stronger and more flexible in more ways than one. You have more yogic maturity. You better understand the what's, why's, and how's of your practice. And your choice to reintroduce yourself to the pose is a good one which means you are more inclined to be content with the outcome of your efforts.
Sometimes it is your attachment to the pose which prevents you from experiencing it. Perseverance, commitment, and boldness are valuable yogic traits, but too much of a good thing turns virtue into folly. Sometimes wanting something too badly turns you into your own biggest obstacle.
Vairagya is the act of choosing your higher good, of setting aside feelings of success or defeat, and of neutralizing temptations. It is learning to be in the moment exactly the way it is without measurement or comparison or judgment. It is a kind of trust in yourself that you are right and good and capable. When your practice is free from the burdens of conquest and validation, it has the capacity for exponential growth.
Of course sometimes the right choice is to push through with dedicated effort. And sometimes skipping a pose is more about avoidance than intelligent discrimination. And sometimes taking a break doesn't give you any more capacity than if you had stuck with it. This is just one way to open your practice to new experiences, not a guaranteed liberation of obstruction.
But I do know from first-hand experience that it is worth considering. If you are struggling with a pose, maybe your yogic efforts are better directed elsewhere for the time being. In this way, you can continue to strengthen your practice as a whole while establishing yourself in nonattachment. By letting go of desire, you might get exactly what you want.
The following is the "16th and 17th week" sequence in its entirety. The asana in blue correspond with the list of "Important asanas in Course I" on page 468 of LoY.
Sanskrit names are in bold.
(English names are in parentheses.)
[Numbers in brackets correspond with illustrations.]
Poses with an * are new to the sequence.
Observe the change in the order of the asana.
Salamba Sirsasana I (Supported Headstand first variation) 
Utthita Trikonasana (Extended Triangle pose) [4 and 5]
Parivrtta Trikonasana (Revolved Triangle pose) [6 and 7]
Utthita Parsvakonasna (Extended Side-angle pose) [8 and 9]
Parivrtta Parsvakonasana (Revolved Side-angle pose) [10 and 11]
Virabhadrasana I (Warrior pose first variation) 
Virabhadrasana II (Warrior pose second variation) 
Virabhadrasana III (Warrior pose third variation) 
Ardha Chandrasana (Half Moon pose) 
Parsvottanasana (Intense Side-stretch pose) 
Prasarita Padottanasana I (Standing Wide-angle forward bend first variation) [33 and 34]
Prasarita Padottanasana II (Standing Wide-angle forward bend second variation) [35 and 36]
Padangusthasana (Standing Big-toe pose) 
Padahastasana (Hand-under-foot pose) 
Uttanasana (Standing forward bend) 
*Urdhva Prasarita Ekapadasana (Standing Splits) 
*Utkatasana (Fierce pose) 
Parighasana (Gate pose) 
*Ustrasana (Camel pose) 
Salabhasana (Locust pose)  or Makarasana (Crocodile pose) 
Dhanurasana (Bow pose) 
Chaturanga Dandasana (Four-limb Staff pose) 
Bhujangasana I (Cobra pose first variation) [73)
*Urdhva Mukha Svanasana (Upward-facing Dog pose) 
*Adho Mukha Svanasana (Downward-facing Dog pose) 
*Virasana (Hero pose) 
Salamba Sarvangasana I (Supported Shoulderstand first variation) 
Halasana (Plow pose) 
Karnapidasana (Ear-pressing pose) 
Supta Konasana (Reclined Angle pose) 
Parsva Halasana (Side Plow pose) 
Ekapada Sarvangasana (One-legged Shoulderstand) 
*Parsvaikapada Sarvangasana (Side One-legged Shoulderstand) 
Jathara Parivartanasana (Revolved Abdomen pose) [274 and 275]
Urdhva Prasarita Padasana (Upward Extended Legs pose, aka UPP) [276 to 279]
Paripurna Navasana (Full Boat pose) 
Ardha Navasana (Half Boat pose) 
Janu Sirsasana (Head-toward-Knee pose) 
Paschimottanasana (Seated forward fold) 
*Purvottanasana (Upward Plank pose) 
*Siddhasana (Adept's pose) 
Ujjayi Pranayama without inhalation retention (Section 203) in Siddhasana