Normally you go to class, do some home practice, put in the effort and you notice improvement in flexibility, strength and technique. Wonderful stuff! However, this is subject to change. There has to be an underlying mental reorganizing to support ongoing physical growth. It is a bit like the larva having to undergo metamorphosis in the chrysalis before emerging as a butterfly. There has to be a period of 'breaking down and re-configuring' of the deep-seated aspects of ourselves before we can move ahead.
Think of an arrow being pulled back in order to fly to its destination. Imagine if the arrow said “no, no I am going backwards”, not understanding it is actually preparing for a major movement forward. You can't go forwards without first going backwards! Contrary to popular belief, yoga is not about green meadows and unicorns. The real thing is challenging work and you have to actually put in the time and effort to elicit change. You know the saying “you have to pay your dues to sing the blues”.
We are amazingly complex beings and that is one reason why there are so many yoga poses, with each addressing a specific aspect of the mind. I sometimes think how we can liken the body to a yoga block and the individual postures are like drilling through it at different angles, until the block (representing restriction) is removed or erased and then we have freedom of movement in the body and mind.
As you progress you cannot completely let go of the lower level practices, because if you do, you will lose them. In the book 'Light on Yoga', BKS Iyengar says in the Advanced Syllabus, you can now stop doing Standing Postures and just focus on the higher-level asanas. From personal experience, I can tell you this is not true! If you don't use it, you lose it. I’ll tell you up front, do not stop doing the lower level practices as you progress. Say you have progressed to an Intermediate Level, don't think for a minute you can forget about the techniques that got you there in the first place. You still have to practice them, just not as often but at least once per week. Remembering ideally we practice six days per week (class and/or home practice).
Nicky and I have had some pretty amazing experiences over the years. It is not uncommon for students who have attended teacher training to go away, drop their practice, and then the next year apply to do the next higher level. This doesn’t make a lot of sense, because by dropping their practice they are not even capable of doing the first level, much less something more intense, difficult and potentially harmful.
Not everyone has the time to fit in six days of practice each week, unless you are clever. Go to bed earlier and get up earlier - there is always time for Sun Salutations. Book your practices into your diary as appointments, exactly the same as you would for a dentist, doctor or hair cut. Some practice is better than no practice. If you get on your mat then make sure you congratulate yourself, because it is not easy and takes discipline so celebrate when you do.
It is a common misconception to think you cannot regress as you certainly can. To your benefit is body memory and what you have uploaded into your nervous system will be there for a long-time, but physical strength and control over your body dissipates quickly. If you have had a break for some reason (and I am sure it is valid), don't think you can safely or comfortably jump back into the same practice you left off at. I tried it (over a break of 7 weeks) and I could hardly move for a few days after I returned to practice. It felt like there was ground up glass in my muscles and joints.
Regarding progression, how do you know when you are ready to move up? My recommendation is when you are comfortable with the techniques, understand the terminology, can execute the postures competently and have started a home practice, then you are ready. But, as discussed above, don't drop your current practice as you move into the higher levels. It would be sensible to discuss the intended move with your teacher. If you have been diligent in class attendance then your teacher will be in a good position to advise you. It would also be a good idea to book into a private one-on-one class/es to go over any weak areas and get help with specific techniques you are not 100% clear on.
You cannot conquer yoga. You can align your body/mind with the techniques and practices. You can harmonize your lifestyle with the underlying and supporting ethics. It is not a matter of dominating your self, but of liberating your SELF. To do this is the essence of yoga.