We have to ask: how come this widow spends all her time talking about grass and trees and flowers? Shouldn't she be talking about her dearly departed darling? Well, as it turns out, "The Widow's Lament in Springtime" is all about our speaker's husband's death. It's just that her grief is in disguise. In this poem, we can tell exactly how much pain this woman is in, based on her descriptions of the natural world around her. How she relates to nature tells us how she is coping with her husband's passing. And she's not coping well.
Our speaker is just projecting her inner emotions on the outside world. She resents nature, because nature keeps on going while her husband has died.
Nature is the solution to this woman's sorrow, not the cause of it. While she resents nature for continuing to bloom while she withers away in grief, if she would only get outside and "fall into those flowers," she might be happy again.