Four short lines, and almost every word is doing quiet work. The speaker is afraid of a moment that hasn't happened yet: a night when someone wakes up and realizes they miss the speaker. The fear isn't really about being left. It's about being missed too late, when the longing finally arrives but nothing can be done about it.
The speaker sounds hurt, but also resigned. There's a hint of warning in "I fear," as though they're gently telling the other person what's coming, yet there's no anger or begging. The break after "far" makes "Too late" land heavily, like a door closing. The poem doesn't say why the speaker will be out of reach, and that gap is part of what makes it haunting. The mood is sad, hushed, and a little accusing, the feeling of someone who already knows how this will end and is saying it out loud anyway.