Shunt Voltage Reference LM431

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OK, that's a start, and in that circuit what's supposed to be
happening is that if the output voltage rises above 90V and the Zener
is soaking up 82V of it, regardless of the current through the Zener,
then that rise in voltage will force more current through the LED in
the opto, which will cause it to emit more light, which will cause the
transistor in the opto to turn on harder, which will cause the
controller to do whatever it has to to lower the output voltage.

Not trying to be insulting, but from my point of view the problem
seems to be that you don't quite have a grasp on what's required to
make the circuit work and, consequently, you go flitting from post to
post, changing the ground rules, and trying desperately to find a
solution you can understand.

If you can settle down and choose a topology, perhaps someone here
might be able to help. :-)