Memory

It's not just about forgetting your keys. Memory—in this poem at least—can be a really painful and dangerous thing, especially for the speaker who wishes to "hasten down to that tomb" with her lover. So memory, like those "tears of useless passion," is something that needs to be checked and controlled so that the speaker can go on living, in spite of all her grief.

• Lines 3-4: The speaker worries that she forgot to love her lover and that time's severing wave has gotten in the way again. So we immediately sense that memory is susceptible to time in a bad way here. The more time that goes by, the less you remember. 
• Lines 5-6: The speaker's thoughts no longer hover over her lover's grave and she's kind of concerned about that. She feels a bit vulnerable to the passing of time and what it's done to her remembrance.
• Line 11: Nothing proves faithfulness better than a "spirit that remembers," and this was particularly true for folks in the Victorian era. A widow was expected to remain faithful to her lover even after death, so remembering is mighty important here.
• Lines 13-14: The "world's tide" is always bringing new changes that will affect the speaker's memory, especially when recalling her "Sweet Love of youth." That's not to say she loves him any less, but growing up tends to put a new perspective on those young desires.
• Line 30: Memory's "rapturous pain" is depicted here as a sort of indulgence that must be checked for the speaker's own survival. It's kind of like a drug that puts her in a state of rapture even though she knows that pain isn't doing her any good.