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For example; is your body size and shape the same as the instructors?
 
If not you may find it hard to keep up, or achieve the positions demonstrated.
My own aging body can no longer do what it used to do for me, regardless of all the will-power in the world. I now have to modify what I do to keep in tune with what my body will benefit from the most.
 
There is NO point (it is completely counter-productive - and dangerous in the longer-term) to force yourself to try to keep up with,
or mimic, what other, perhaps fitter,
younger, leaner, people can do.

 
Remember to always work-out within the constraints of your own body, rather than try to force it to go beyond its current limitations.

Of course, the more you work-out and the more effort you put in, the easier everything becomes to do.