不是忘了我们的约定而是忘了我们的曾经
[bù shì wàng le wŏ men de yuē dìng ér shì wàng le wŏ men de céng jīng]
Translates to 'It's not that I forgot our agreement, but I forgot our past.' This suggests a sense of moving on where memories of agreements still hold some importance, while past emotions or events have faded.