First of all, there are more words to describe coffee than there are ones to describe wine, bitter should never be one of them. I don’t drink my coffee black either. That being said, good quality green beans that are properly roasted and then ground and brewed within a very short amount of time should have a very clean and fresh taste. Time and exposure to air are two things that will turn any coffee into a very unpleasant experience. At Berries To Beans we package our coffee as soon as the beans have cooled in sealed bags with one way valves. The valve allows the coffee to release co2 gases as it reaches its peak flavor without allowing exposure to air. Once sealed in these bags the coffee will remain fresh for a quite some time until opened. The coffee reaches it full sweetness and flavor about 24 to 48 hours after it is roasted.
Here are some other factors that could cause coffee to be bitter. Coffee left on a burner. This will literally cook the coffee and give it a sour and bitter taste which no amount of milk or sugar will fix. If possible use a brewing system that brews directly in a thermos and does not sit on a heating element.
Another reason could be too fine a grind. Many people think they can get more mileage out of the coffee by grinding it fine and using less, not true. The grind should be determined by the type of brewing system used. The most common type is an auto drip. If you are grinding your own coffee set your grinder for that type of brewing system. Coffee that is brewed too strong will also be bitter. If you like your coffee strong use a dark roast and a normal amount of coffee. Increasing the amount of a poor quality, stale coffee will only result in a poor quality, stale tasting and bitter brew.