I can't understand what I think on earth,I just know I am sad but can't do anything to cheer up.

I can't understand what I think on earth,I just know I am sad but can't do anything to cheer up.
I can't understand what I think on earth,I just know I am sad but can't do anything to cheer up.

I can't understand what I think on earth,I just know I am sad but can't do anything to cheer up.
我不明白我究竟在想什么,我仅仅知道现在我很难过不知道如何才能振作起来.

我不能明白我认为在地球上，我只知道我很伤心，但我却不能做任何事来振作起来

我不明白我究竟在想些什么。我只知道我很难过，但是又不能做任何事让自己开心。（你的原句最后应该是cheer up 吧）on earth 引申义表示“究竟” cheer up 表示兴奋 ，使高兴。