We are sad to hear you didn't like the Mango Black tea!
There are a few reasons tea can become unpleasantly bitter:
1 - Steeping at too high a temperature, or
2 - Steeping for too long. The longer you brew tea, more tannins are released causing the bitter taste.
(It's also possible that if your water is heavily chlorinated that it could make tea taste bitter as well, but let's assume you were using filtered water.)
Another cause of a bitter tasting cup is using too many leaves. A good rule of thumb is one teaspoon of tea per 8 oz of water.
If you follow these suggestions, it should bring out the flavors that you were looking forward to! But each tea drinker's taste is different, and this particular tea may just not be right on your palate.