Switchable regulator with a Zener diode
Your circuit is completely confused, sorry, and well beyond repair. 

Here is an approach that will work: 



You can (say) use 1.68K for R2 and parallel R1 = 1K with a MOSFET and 1.21K resistor. When the MOSFET is on, Vout = 5V, otherwise Vout = 3.3V. 

You can modify a very inexpensive buck regulator module to get this to work (remove the pot and resistor and replace with the R1/R2/R3/R4/MOSFET), and I highly recommend this approach., 

Buy a bunch of them because they are cheap and I suspect you might experience the odd problem given from where you are starting. 







simulate this circuit &ndash; Schematic created using CircuitLab