我只知道 我放不下的英语怎么说

I only know that I can't let it go.
综合了2、4楼的答案，因为这样的组合相对来讲最地道。give up是【放弃】的意思，题目给的中文比较文学色彩，用【放弃】显然不妥。

I just know I can't let it go

i just know that i can't give up

I just know that I can’t give up.
希望能够帮你，加油

i just can not put it behind.

I only know ,I cannot lay down