I know this is a joke, but usually it's caused by old cartridges. If you replace the valve cartridge then it will probably mix better. This is usually from old/bad seals. This is why new valves mix fine, but as the seals age, mixing performance gets worse.
Pulling old cartridges can be a pain in the butt so probably best to call a plumber if you don't like diy adventures.
Are you new to separate taps? I'm assuming you mean one hot one cold. I turn on hot halfway then turn on cold like a quarter of that. Then mess with the cold only from there
Hope that helps!
First of all, the comfy range for humans is small and it is not even. E.g. most people will find around body temperature of 37°C nice. But 5 °C above that is burning hot, while 32°C is more lukewarm, than cold.
Then the cold water temp is probably between 10 °C and 15°C depending how long it stood. Same of the hot water temperature, where the first bit in the pipe is more like 20°C and then the boiler comes pushing 60°C in.
So the temperature range that gets mixed changes drastically in the beginning and small changes can make for a large difference in the "center" of the mixing, which is also where the comfy temperatures are.