Why is it always bitter :(

I use a 1-cup and 3-cup Moka pot. I boil the water first, fill right up to below the little silver knob thing, put the grounds filter in, I use Cafe Bustelo grounds and fill it. I screw on the top. I start at medium heat until it starts brewing then I lower the heat to the lowest setting until it starts to sputter.

The coffee is bitter almost every time. What am I doing wrong?