The speaker has just realized they're in love, but the realization comes too late to matter. The forget-me-not, a flower tied to remembrance and faithful love, becomes a bitter image here: the speaker finally has it, and it's "worthless now." Something seems to have changed for the person they love, and the poem suggests that person is now happy, perhaps with someone else or simply in a way that leaves no room for a confession.
The speaker chooses silence. They admit that a more selfish person would speak up, but they can't bring themselves to disturb that happiness. Swallowing petals until they "wither up along with my heart" is a painful picture of holding feelings in, and it makes the silence feel physical, like something choking them. The short last line, "I pray you keep being happy," is quiet and generous, though it hurts. The mood is sad and tender, with love and grief tangled together and no hope of resolution.