The speaker can't stop thinking about a person who has hurt them. People tell them to move on, and the speaker's answer is simple and honest: they would, if they could. Even when this person is unkind, they stay on the speaker's mind.
The twist comes in the second half. The person the speaker is longing for is preoccupied with someone else ("She's all that you think about"), yet still feels unsure about her. He then turns to the speaker for advice about this other woman. That is a painful thing to be asked when you still have feelings for him.
The speaker's reply, "I'm sorry, Sir. / I can't help you there," is short and polite, but it carries a lot. It works as a refusal, and it also hints at hurt the speaker won't spell out. The mood is quietly sad, with a little dignity in the way the speaker holds back. The simple rhymes (should/could, mind/un-kind) give it a plain, almost spoken feel, which makes the ache seem more honest.