It's enough to make a girl depressed. Three years after it was filmed, ''Prozac Nation'' still hasn't hit U.S. theaters (although it did recently open in director Erik Skjoldbjaerg's native Norway). ''We're in the process of determining a release date,'' a Miramax spokeswoman explains cryptically. ''I'm resigned to it,'' says Christina Ricci, who coproduced and stars in the adaptation of Elizabeth Wurtzel's 1994 psychopharmacological memoir. Jason Biggs, who costars as Ricci's boyfriend, offers one reason for the movie's delay: ''I just don't know that the center of the story is a very endearing and likable character.'' At least the project warmed up Biggs and Ricci for their love scenes in Woody Allen's upcoming comedy, ''Anything Else.'' According to Ricci, ''all the sex stuff was really easy and comfortable for us.'' Who says Prozac decreases your libido?