Apple Cider can be turn out bitter because of a variety of reasons. Here are some to help you out:
Low Residual sugar. Sweetness is the best mask for bitterness (for example coffee and chocolate). Try using some wine stabilizer and back-sweeten your cider. Excess Tannin: Adding too much oak chips, fruit skins, and other sources of natural tannin can introduce astringency, woody feel, or bitterness in your wine/cider. The best way to reduce is by using a fining agent. Isinglass, Bentonite, and Gelatin can help you control excess tannin. Aging: Time mellows down all harsh flavors in cider/wine. Try aging your cider/white wine by 1-6 months and red wine by 1-3 years. With time the complex biochemical reactions will make your beverage more flavorful and amazing. Problem with the fruit: Although you can make cider/wine from any fruit, some fruits are not that flavorful. When you take the sweetness out (by converting it to alcohol/vinegar), the residual flavors start emerging. Try to experiment with a different harvest or fruit type. Mixing two or three different fruit also helps.
For more details refer to our off-flavors guide.