We hate to be the bearer of bad news, but Narcissus is dead. No, not narcissism. That's still alive and kicking. We're talking Greek mythology here, Shmoopers. Echo-the-nymph's number one crush Narcissus has passed away, and "Slow, Slow, Fresh Fount" is all about that death. But it's also about Echo's own deadness – she feels "dead" because she no longer has Narcissus and is sadder than sad about the whole ordeal.
In this poem, death isn't really death, but rather rebirth. Narcissus is dead, but he lives on as a "withered daffodil," a pale and incomplete (but not totally dead!) version of himself.
Echo is still alive, but she is also nearly dead. She is so consumed with sadness that she can't think about anything but death and she can't do anything except mourn. That's no kind of life.