Discrete linear voltage regulator
I'm going to steal this schematic from these posts:
Basic Op-AMP as voltage regulator

Does this voltage regulator use &quot;on-off&quot; control, or is it a voltage follower?

So you can see that your circuit has numerous issues:

Your zener reference has no power supply.

Your zener would fry even if it was connected to a power supply because there is no current limiting resistor.

Your MOSFET is backwards. You said the smoothing cap is connected to the drain of your MOSFET. This is correct, however, what you have drawn in your schematic is the source terminal connected to the smoothing cap.

You have no negative feedback.

Your zener reference is connected to the wrong input

Your divider is connected to the wrong input (your attempt at a divider anyways...it's missing a connection to ground so its not actually a divider as you have drawn it).

Your zener reference is already larger than your desired output voltage, yet you still step down your actual output voltage before comparing it against the zener reference. You either:


a) have your reference be equal to your desired output and compare your actual output against the zener directly (no divider in the negative feedback loop)
OR
b) have the reference be smaller than your desired output, and step down your actual output with a divider before comparing it against the zener. Then your desired output is equal to the inverse of the divider ratio multiplied by the reference voltage. That is because when your actual output equals your desired output the voltage being compared against the reference (the actual output voltage which is stepped down and then fed to the input of the opamp) actually matches the reference voltage.
I also no decoupling capacitors for the opamp.