Person A is a player known for their 'flings', who genuinely falls for Person B. Some asshole lies and tells Person B that Person A cheated on them.
Person B: And here I was, thinking I was different. Thinking that I'm not just one of your 'phases'.
Person A: They're lying, I promise.
Person B: Prove it.
Person A: I can't. But... I- I love you, and I don't want to lose you...
(Bonus points if it's their first time saying 'I love you'.)