Our speaker is no longer on good terms with desire, at least in the romantic sense of love and sex and spring flowers. In fact, in her disconnect from the world, she seems to really only have one desire – ending her isolation.
Lines 2-3: Fire is a pretty well known symbol for passion and desire. So when our speaker creates the metaphor "the new grass / flames," desire is brought to mind. Come to think of it, spring has an equally strong association with love and desire, so having the grass be new reminds us that this is spring, and further cements the association with desire.
Line 5: Now that symbol of desire is used to show us how our speaker no longer actually feels desire. It has been replaced by a cold isolation. If a fire isn't hot, is it still a fire? We're not sure. If a life doesn't have any desire or liveliness, is it still a life?
Lines 27-28: These last two lines, with their parallel structure, give us a pair of images which are not only suggestive of suicide, but also symbolic in terms of reuniting our speaker with the natural world. Using that preposition "into" in both lines really emphasizes our speaker's desire to enter (or re-enter) the natural world, so she can escape the grief-filled world in which she now finds herself.