Well mystery solved. I could have SWORN that I didn't, but last time I changed the thermostat, I put a fail-safe one in there. I had a problem with my fan and the engine overheated a little, so the thermostat was locked open. So, that's why.

Also, it turns out my radiator has had a slow leak for a long time and was low on water. It just sprang a leak somewhere for no apparent reason. It's an aftermarket all-metal radiator (brass I think or bronze or whatever they use). Well, luckily I still had the old aluminum radiator which had nothing wrong with it lol... I swapped it back in. I also changed how my SPAL fan-pwm v3 electric fan controller was wired up... Instead of using the temperature gauge sending unit to get the temperature reading, I switched it so that it uses the sensor that the ECU reads. That way I can tune when the fan goes on to be more in tune with the ECU's readings. I found that the temp gauge sending unit's readings didn't coincide very well with the readings from the temp sensor the ECU uses.

Everything's working great now!