Tibetan Tai Chi Gung: Exercises 2 and 4 are grueling, Exercise 1 and 3 are a breeze

I find Exercise 2 and 4 to be very demanding on me. I guess I’ve spent 50 years never involved in circular movements where the center of the circle is an area just in front of the spine.
I have been forced to only rotate 1-3 each inches in each direction and stop far short of a full turn. In fact, the spinal tuck greatly reduces the amount of turn you can do. So if I maintain the integrity of spinal tuck for the duration of the turn then there is very little turn possible. But if I drop the spinal tuck then I can turn more.
The spinal tuck makes it much harder to stand up. But as you’ve said, most people are not actually using their legs/lower body to stand and move.
I suppose this is basically a re-orientation of how I move and over time it gets less demanding?
— Terrence

David responds

My intuition says you’re tucking past your current comfort level, then locking it in place and trying to force it.
Release the muscles as much as you can, don’t worry too much about how far you tuck. 
The fluidity of motion is more important right now for you than a specific posture forced on the body.