I think that the most important question you have to ask yourself is, do you still enjoy teaching? If the answer is yes, then enjoy it, and let the students progress at their pace.

You are an instructor. That alone means that you trained harder and still train harder than the average student, so do not get upset that some are not capable of making the higher grades. For many it is a hobby, fitness workout, social time. Their aspirations may not be to be the next instructor, but just be the best they can be within the time that they can afford to set aside for that activity.