Missing someone and regretting a mistake are tangled together here. The speaker describes days when emotion takes over the whole body: the heart pounds, the eyes "sweat" (a blunt, slightly awkward way of saying tears), the knees shake, and even memories seem to "jog," as if they can't stay still. Every one of these sensations leads back to a single person, "you," and the feelings only grow stronger, "double," when the speaker thinks of them.
The key confession comes near the end. The speaker says they wish they hadn't "picked rocks over you." The poem never explains what the rocks are, so it's up to the reader to decide whether they stand for something hard and cold, or for a bad choice more generally. The speaker blames unstable feelings on that day and closes with a simple, honest line: "I didn't mean to hurt you." The mood is raw, apologetic and a little helpless, made more convincing by how plain the language is.