Fortunately I acted on the impulse of the moment.

幸运的是，我是一时冲动行事的。

" Clearly he wrote the letter keeping himself well in hand, but at the end his self-control snapped and, on the impulse of the moment, he scrawled those four desperate words" .

“明明他把信写得很好，但最后他的自制力崩溃了，一时冲动，他潦草地写下了那四个绝望的字眼”。