If I do the Moderato C Singing series (or similar) I notice that when I have to play softer after the solid, loud C I feel I can not make the notes unless I make my lip aperture smaller. So what to do: should I start off with a smaller aperture or should I learn to make the aperture smaller (not pinching) going up?
You may need someone to look at your embouchure. I think the width of the aperture gets slightly larger when more air is pushed through to increase the volume. If you have the correct tension at the aperture corners I think the aperture will reduce slightly when the airflow is reduced. It may need control (rather than adjustment) of the tension. There should be no resetting required.
Do you have a visualiser and can you see what’s happening?