A person who has been left for someone else tries to shut down any claim of love from the one who walked away. The speaker doesn't shout or list grievances. Instead, they state a simple piece of logic: if you really loved me, you wouldn't have gone to her.
The poem is short and plain, and that helps it hit hard. There's no fancy imagery, just the bare argument of someone who has worked the situation out and can't get past it. The final line, "would you?", is the sharpest moment. It sounds like a question, but it's really a challenge, and it leaves the other person with nothing to say.
The mood is raw and bitter, but hurt shows through the bitterness. Someone this certain that the love "isn't true" is probably still wounded by it. Wanting to hear "I love you" and refusing to believe it can sit uncomfortably side by side.