There is a constant struggle between these two worlds in Keats' poetry. In "Ode on a Grecian Urn", "Ode to a Nightingale", "The Eve of St. Agnes", and others, characters express disappointment when they compare the ideal worlds formed in the imagination to the necessarily subpar world of reality.
"Romance" is defined here as a movement which prized and prioritized vivid moods and flights of fancy. Keats' constant preoccupation with early death (which proved to be a reasonable preoccupation, in his case) and his focus on the impossibility of achieving artistic/romantic fulfillment during his lifetime are interests that both fall squarely within the Romantic tradition. In his treatment of such themes, he was greatly influenced by another Romantic poet, William Wordsworth, who described the moments of drama and insight that can occur in the course of everyday life.
The Natural World
As a true Romantic, Keats shows extreme appreciation for the natural world in his poetry. Detailed descriptions of plants (including over a hundred species names) are included in many of his poems, including "Ode to a Nightingale" and "Ode to Psyche". For Keats, the natural world represents a kind of Eden, and is the only environment which comes close to mirroring the idyllic world of the imagination.
The Nature of Beauty
Keats states, in a letter to Richard Woodhouse, that "the mere yearning and fondness" he has for the Beautiful is the greatest impetus for his poetry. But the nature of beauty itself is something Keats also explores in a few poems. In "Ode on Melancholy", he expresses the Romantic idea that beauty and joy can only be found in opposition: beauty can only be found in melancholy. "Ode on a Grecian Urn", however, concludes with the timeless lines, "Beauty is Truth, Truth Beauty," perhaps a simpler concept and a more reassuring answer. In any case, a love of beautiful things -- both physical and philosophical -- is one of the touchstones of Keats' work.
Any casual reader of Keats will quickly recognize that mortality is one of the poet's major preoccupations. Having lost his father when he was at age eight, his mother at fifteen, and his brother at twenty-three, Keats was forced to reckon with the human condition from an early age. The topic of mortality arises directly in poems such as "When I have fears that I may cease to be", in which Keats struggles with the prospect of dying before he has achieved artistic success. Similar themes are addressed more indirectly in "Ode to a Nightingale", in which he wishes (though perhaps only for dramatic effect) to die while listening to the songbird of the title.
The ultimate inextricability of pleasure from pain, joy from sorrow, happiness from melancholy, and life from death is essential to Keats' poetry. Keats believed that recognition of these relationships is a hallmark of practical education. In a letter to his brother George and sister-in-law Georgiana (in May of 1819), Keats writes, "Do you not see how necessary a World of Pains and troubles is to school an Intelligence and make it a soul?" Without pain, pleasure could never be experienced fully. This paradox is described most directly in "Ode on Melancholy", but also appears in "La Belle Dame Sans Merci" and "The Eve of St. Agnes".
Romantic Notions of the Female
Mystical female figures are a constant presence in Keats' poetry: these range from Psyche in "Ode to Psyche", to the Belle Dame in "La Belle Dame Sans Merci", to the female figures of Love, Ambition, and Poesy in "Ode on Indolence". Female figures are often marked by a dreamy otherworldliness; in fact, even mortal women like the mistress in "Ode on Melancholy" have "peerless eyes" that are to be "fed upon" in the midst of angry moods. Madeline, in "The Eve of St. Agnes", represents notions of female virginity and sexual purity. Keats's male figures are often chivalrous, but Keats himself often plays the role of spectator-poet in relation to the women he describes.
Keats’ Poems and Letters Questions and Answers
The Question and Answer section for Keats’ Poems and Letters is a great
resource to ask questions, find answers, and discuss the novel.
I'm sorry, you haven't noted the anthology or which particular poem you're already working with. Gradesaver has a long list of summaries and analysis for Keat's work. I have provided the direct link below.