The speaker is ending a relationship, and the strange part is that they insist it's out of love. Their partner believes in them and in the couple, but the speaker doesn't share that belief. They are convinced they will eventually let the other person down, and they say it "happens every time," which suggests a pattern they've seen before and can't trust themselves to break.
What stands out is how the speaker frames the choice. A painful goodbye now, they argue, is kinder than a betrayal later. The line "You're the best thing in my life, I'm the worst in yours" shows how low their self-image has sunk. Whether that judgment is fair is left open, and the partner clearly disagrees.
The mood is sad, apologetic, and a little resigned. The short, plain statements sound like someone who has already made up their mind. The final line, which admits the partner won't agree "now," hints at a hope that time will prove the speaker right.