《忠心耿耿》

Actually there are no grammar or other mistakes in you translation，but we can say it in a graceful way.

你在那个时刻给予我那无法替代的感觉。

你若不离，我必不弃。

We use these sentences more often in poetry and novel，but your translation is more better used in our daily conversation.

I thought I'd try to add my 2 cents...

《忠心耿耿》

这一特殊的时刻，你带给我一种忠心耿耿的样子，别人却不以为然。只要你坚持，我保证和你就始终不离不弃。

《忠心耿耿》

I don't think there is something should be corrected, what you wrote is absolutely 100 percent correct, I don't understand why people replaced some of the words with their own thoughts. Of course, maybe it can be better, you can make it more like a poetry or something, but grammatically, I don't see any part that is improper and I couldn't even tell that Chinese is not your first language.  Great work, Lucy!

《忠心耿耿》

在那特別的一刻, 你給了我別人無法給予的感覺.

我能保證, 只要你堅持, 我將永遠不會離開你. (或 我將不會離你而去)