You're probably doing these three things wrong with a Moka Pot.
First, grind. Just slightly coarser than espresso. Too fine and the pressure builds up too much. Too coarse and you get weak, watery, underextracted coffee. You want something between espresso and drip. Think coarse table salt.
Second, fill the bottom chamber with water that is already hot. Do not cover the relief valve. That little nipple on the bottom chamber. Cover that and you are going to blow your shit smooth off. The water you put in there should be around 200 degrees. A kettle is fine. This shortens the time on the burner and keeps the coffee from getting burnt, bitter, and metallic.
Third, medium to low heat. Leave the lid open so you can watch it. When the coffee turns light brown and starts sputtering, pull it off the heat. The sputtering means you are extracting bitter compounds you do not want. Pull it early. Let the residual heat finish it.
To slow that extraction down even more, run the bottom chamber under cold water in the sink. That stops the leftover heat from pushing those flavors into the cup.
Bonus. Put an AeroPress filter on top of the coffee bed. That keeps the grit out.
And that is how you make damn good coffee at home with a Moka Pot.
Here is a link to video to watch about this.