Why is there a need for a negative (and stabilized!) voltage? As is the circuit, just put an additional diode in series with the D7... By the way, what is the primary role of this diode and resistor R5?
This is a variable voltage power supply without regulation, so there is no sense to try to adjust it with precision, at any value, especially at the lowest range. Imagine connecting to its output a 5V or 3V device and expecting to survive!
The circuit seems to have "designed" with the random felling of parts on the table!!!
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A simple way to implement this is to remove all electronic components of the circuit, keeping the transformer, rectifier and filter capacitors ... place a variac to the mains' side. Doing so, you will have better results in terms of stability of voltage and much more, the efficiency.
Otherwise, consider a switching power supply.
I you insist to design a linear power supply, start studying from what is each component, its properties and how it can be used with other components in a circuit. An about 1kW circuit is not a good start for a newcomer to electronics.