There's a big difference between tragic and bittersweet.
Bittersweet: the good guys won, evil is vanquished, but the heroes had to pay a steep price to overcome evil (maybe the land is saved, but the main's love interest/true friend is gone forever). It's important to note that the heroes' main goal has to be achieved - if the love interest is absolutely central to his quest and happiness, and that love interest dies, the ending can no longer be called bittersweet.
Tragic: nobody wins. Evil is vanquished and the good guys triumph, but in the process have negated everything that they once fought for, leaving their entire quest pointless and void of meaning. Or maybe they realize that the "villain" was actually a good guy all along, and as they kill them, they realize that they really should've let them accomplish their plans. Their quest actually left the world worst-off than it had started.
These are just examples, of course, but there's a fine line between bittersweet and tragic, IMO.
--
When you have nothing to say, quote yourself ~ Solfadore