Your question seems to infer that the two states of being a Romantic and a fatalist are incompatible. I would actually disagree. Let us remember that being a Romantic (with a capital "R" as opposed to a lower case "r") does not mean that you are a romantic in the way we think of this word today. Rather, it relates to a philosophical and literary approach to life that values intuition over reason and looks to nature for inspiration and restoration. There is no sense therefore in which being a Romantic is incompatible with being a fatalist. Keats, as this poem shows, was very realistic about the fate of humans and the suffering and oppression that characterised life. Note how he compares and explicitly contrasts this with the eternal and immortal beauty of the nightingale's song:
No hungry generations tread thee down;
The voice I hear this passing night was heard
In ancient days by emperor and clown...
Keats therefore was equally very aware of the problems of human life and recognises that it was characterised by suffering and pain and death, but he was also able to look to nature in search of beauty and of something transcendent, as this meditation on the beauty of the nightingale's song clearly shows.