我以为我在你心里无人代替你在我心里本来就无人代替
[wŏ yĭ wéi wŏ zài nĭ xīn lĭ wú rén dài tì nĭ zài wŏ xīn lĭ bĕn lái jiù wú rén dài tì]
This lengthy phrase can be translated as 'I thought I was irreplaceable in your heart, while in fact, no one replaced you since you were never in my heart'. It expresses regret over false assumptions about a relationship and a confession that the other person was never truly loved.