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Over-extraction is usually the culprit - that's when the water stays in contact with the coffee too long. Check how long your coffee is brewing: with a filter machine it should be at most 4-5 minutes, with a French press more like 3-4 minutes. Water that's too hot or coffee that's ground too fine can also cause bitterness - try using slightly cooler water (around 90-95°C instead of boiling) or a coarser grind, and see if that helps.
For me it was the water temperature for the longest time - I'd just pour boiling water straight onto the beans and wonder why it tasted so bitter. If the water's too hot, you're extracting way too many bitter compounds. Try it around 90-95 degrees, that makes a huge difference.
Wait, I wouldn't just blame the usual suspects! Sure, overextraction and water that's too hot play a role, but often it's the coffee beans themselves. How old is your coffee? If it's been sitting around for a while or wasn't stored properly (so not airtight and protected from light), it oxidizes and gets bitter. Even the best brewing method won't save it then!
And one more thing: How fine are you grinding your beans? If the powder is too fine, you're massively increasing the surface area and the coffee extracts too quickly. That's also a kind of overextraction, just not because of time. Try making the grind a bit coarser - not quite as coarse as for a French press, but noticeably coarser than espresso powder. That makes a huge difference for me personally!
And try not using water that's boiling hot either. Around 85-90°C is just more pleasant for most coffees. If you only have hot water from the kettle, just let it cool for a minute before you pour it in. For me, that together with better beans and the right grind completely got rid of the bitterness!
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