What regulates the nuclear fusion rate within stars?

While Centauri Proxima is just 1/8th the mass of our Sun, Centauri Proxima is a main-sequence, red-dwarf star that will burn for trillions of years while our Sun will consume all its hydrogen, begin gravitational collapse and, with the rapidly rising temperatures that collapse triggers, transition into a red giant powered for a short time by fusion of increasingly heavy elements in just 8 billion years. Our sun’s red giant phase will last a relatively short time before it consumes all its fusionable elements, and without the pressure generated by fusion reactions it will collapse into a slowly cooling brown dwarf.

Some supermassive stars consume all their hydrogen, begin gravitational collapse and with their great mass, become so hot they explode as a supernova in relatively short timeframes; whereas others of similar size burn cooler and will last far longer. Stars aren’t like nuclear power stations, with control rods that can raised and lowered to manage their reaction rate. What then controls the rate at which a given star fuses its hydrogen atoms into helium during its main sequence phase?