I had walk'd on at the wind's will,—
In this line, the speaker personifies the wind by indicating that the wind has a will. This personification shows the wind's importance when it comes to the direction in which the speaker walks as well as the complete passivity of the speaker. He lets the wind make the decisions for him and he is simply along for the ride. The speaker gives the wind complete control over his movement.
Between my knees my forehead was,— /My lips, drawn in,
We are given more information on the speaker's bodily position in these lines. We know that his forehead is between his knees and his lips are "drawn in," as if he's trying to physically stop himself from crying out in pain. The image of the speaker crouched so that his forehead is between his knees is an evocative portrayal of the deep pain that he is in, almost as if he were in the fetal position. It emphasizes his vulnerability and weakness in this moment.
The woodspurge flower'd, three cups in one.
The speaker offers a vivid description of one of the weeds that are in his line of sight. The woodspurge’s flower is unique because it comprises three cups. The cups refer to the calyx of the flower.
"Three cups in one" might also be a reference to the Holy Trinity, like how St. Patrick describes the Holy Trinity with a three-leaf clover. Like the three-leaf clover, a woodspurge has three separate parts, yet it is a single entity, just as God has three parts even though he is one entity: Jesus (the son), the Holy Spirit, and the Father.
