Every line here begins with "Without you," and that repetition is the poem's whole engine. The speaker imagines a world with someone missing, and the picture grows steadily darker. It starts light, almost playful: spring wouldn't be "springy," loneliness would be "lonelier." Then the tone shifts. Words will "break" the speaker, memories will "come flooding back," and the final line states plainly that living would be impossible.
What the speaker feels is dependence, close to dread. The other person seems to give ordinary things their meaning, from the seasons to the speaker's sense of self ("I wouldn't be me"). Small details, like scuffing feet, make the loss physical and tired.
The title adds a twist. Throwing supermarket flowers from the windowsill suggests an act of frustration or grief, as if the absence may already be real. The mood is hushed, anxious and sad, and the unbroken pattern makes it feel like a thought the speaker can't stop circling.