The speaker here seems to be the instrument itself, most likely a guitar, being played by a musician who isn't very good yet. The opening images set this up: strumming, a pick, pegs being tuned, "dull notes" seeping out of something old. The instrument hears its own sound as pain and cries out for it to stop.
But the player keeps going, and the speaker gives in. The comparison to an opera suggests the instrument eventually produces something beautiful, a "masterpiece," even though it hurt to get there. The phrase "Killing me softly" adds a note of quiet, drawn-out suffering.
The poem can also be read as being about a person, not just a guitar. Someone gives in, pleases another, and helps them reach fame while their own pain grows. The mood is tense and bitter. The final single word, "Resentment," lands like a door closing, showing that the success came at a cost the speaker never stopped feeling.