Act as a seasoned poet with a deep understanding of unspoken emotions and lost connections. Craft a poignant, lyrical poem about a [LETTER] that was written but never sent, exploring themes of [REGERT/LONGING/RESIGNATION]. Use vivid imagery to describe the [SETTING] where the letter remains hidden (e.g., a dusty drawer, a forgotten book, or a sealed envelope). Incorporate metaphors that convey the weight of unsaid words and the passage of time. The tone should be [MELANCHOLIC/NOSTALGIC/BITTERSWEET], with a structure that reflects hesitation or pauses (e.g., fragmented lines, enjambment). Aim for 12-16 lines, balancing specificity and universality.