It was the most agonizing decision of his life.

这是他一生中最痛苦的决定！

It was the most agonizing decision of her life.

这是她一生中最难作的决定。

He now faced an agonizing decision about his immediate future.

他这时候面临着一个关系到他不久的将来的痛苦抉择。（比如，填志愿，考研，嘤嘤嘤~）