Echo, the speaker of "Slow, Slow, Fresh Fount," must have been one outdoorsy lady. She talks to nature, sings to nature, and uses nature to describe her own emotions. Nature, for its part, is a bit more complicated. While nature seems to participate in human activities (like singing and mourning), there are a number of moments where nature seems to march to the beat of its own drummer. At times, it seems like Echo reads things into nature, or wants to convince herself that nature cares when, in reality, it doesn't at all.
In this poem, Echo and nature are one and the same. There is simply no separating the two.
Echo will never get over her grief because she fails to come to terms with the fact that nature doesn't care about her lot.