Potentiometer is Heating
Common potentiometer are not designed to serve as a load or dissipate power, as in your circuit.

You need a proper dimmer circuit as an example below.



Note that is not isolated and might be hazardous. You rather use an isolated circuit, search on Google isolated dimmer circuit.

Note: You can buy very cheap off the shelve dimmer as well. Although this will only work with incandescent and halogen lamp and not LED light unless they are specifically designed to be dimmed.