Maria, the poem's title and apparent addressee, is someone who once hurt the speaker badly and has now returned. The speaker says she left "pieces" of his heart behind, as if she didn't just walk away but broke something on the way out. Now she wants to put it right, and the speaker watches her "try glue it together."
The word "glue" matters. It suggests a quick, clumsy repair, the kind you use on a cracked mug, not on something alive. The speaker seems unconvinced that she can fix what she broke so easily. The missing "to" in "try glue" gives the line a rough, blunt feel, which suits the tone.
The mood is quiet and bitter, with a hint of weariness. The speaker doesn't shout or plead. He just states what happened, and the plainness makes the hurt feel real. We're left wondering whether a heart, once shattered, can ever be made whole again by the person who broke it.