Harry Styles - Sunflower, Vol. 6 Lyrics Meaning

I feel like sunflower is all about the negative and positive memories of a failed relationship, and how Harry is looking back on these memories after a break-up. "Your flowers just died
Plant new seeds in the melody", implies that the negative memories have now gone away as he has put a positive spin on them, and is trying to "plant new seeds" by turning his pain into something positive. When sunflowers die, you use the seeds from the dead sunflowers to plant new ones, which connects to how he's trying to put a positive outlook on their breakup.