可以用"It has already become a memory"来表达这个意思。这句话的来源可能是因为当某件事物已经成为过去，人们常常会把它想象成一个回忆，因此用英语来表达这个意思时就可以使用这句话。

例句：

The party we had last night has already become a memory. 我们昨晚的派对已经成为了回忆。
The trip we took to Europe last summer has already become a memory. 我们去年夏天去欧洲旅行的经历已经成为回忆。
The time we spent together has already become a memory. 我们在一起的时光已经成为了回忆。