Two short lines carry a whole story of hurt. The speaker says that thinking about someone brings pain that never seems to ease. That sounds like a simple case of missing a person, but the second line complicates it. The pain only stops when the speaker remembers what that person did and why they had to say goodbye.
So the speaker is caught between two feelings. Part of them still aches for the other person, while another part knows leaving was necessary. The memory of the harm works almost like a reminder that the decision was right, though it doesn't make the loss any less painful.
The mood is quiet, heavy, and unresolved. The dash at the end of the first line creates a small pause, as if the speaker is stopping to steady themselves before facing the truth. We never learn what happened, and the poem doesn't explain. That silence suggests the hurt is still too raw to describe fully.