The speaker looks at a half moon and sees it as full. The poem suggests this happens because of loss, and because of something the speaker has "exceeded," perhaps a stage of life or a way of seeing that can't be recovered. The mind fills in what the eyes no longer take in, so the moon looks complete when it isn't.
The feeling is quiet sadness. The speaker's eyes "give up wonder," and the light reaching them arrives "in a sad condition," as if the world's brightness gets worn down on its way in. Nothing dramatic happens, only a slow dimming.
The last image is the strangest and the best. Growing "backward like an ear" suggests that something shaped like half a curve can still feel whole. The ending carries a faint consolation: the speaker is diminished, yet still complete in some odd way. The mood is gentle, wistful and resigned, with a small spark of acceptance.